HMMT November 2017

Find the sum of all positive integers whose largest proper divisor is 55 . (A proper divisor of $n$ is a divisor that is strictly less than $n$.)

Determine the sum of all distinct real values of $x$ such that

$$|||\cdots|| x|+x|\cdots|+x|+x|=1$$

where there are $2017 x$ 's in the equation.

Find the number of integers $n$ with $1 \leq n \leq 2017$ so that $(n-2)(n-0)(n-1)(n-7)$ is an integer multiple of 1001.

Triangle $A B C$ has $A B=10, B C=17$, and $C A=21$. Point $P$ lies on the circle with diameter $A B$. What is the greatest possible area of $A P C$ ?

Given that $a, b, c$ are integers with $a b c=60$, and that complex number $\omega \neq 1$ satisfies $\omega^{3}=1$, find the minimum possible value of $\left|a+b \omega+c \omega^{2}\right|$.

A positive integer $n$ is magical if

$$\lfloor\sqrt{\lceil\sqrt{n}\rceil}\rfloor=\lceil\sqrt{\lfloor\sqrt{n}\rfloor}\rceil,$$

where $\lfloor\cdot\rfloor$ and $\Gamma \cdot\rceil$ represent the floor and ceiling function respectively. Find the number of magical integers between 1 and 10,000, inclusive.

Reimu has a wooden cube. In each step, she creates a new polyhedron from the previous one by cutting off a pyramid from each vertex of the polyhedron along a plane through the trisection point on each adjacent edge that is closer to the vertex. For example, the polyhedron after the first step has six octagonal faces and eight equilateral triangular faces. How many faces are on the polyhedron after the fifth step?

Marisa has a collection of $2^{8}-1=255$ distinct nonempty subsets of $\{1,2,3,4,5,6,7,8\}$. For each step she takes two subsets chosen uniformly at random from the collection, and replaces them with either their union or their intersection, chosen randomly with equal probability. (The collection is allowed to contain repeated sets.) She repeats this process $2^{8}-2=254$ times until there is only one set left in the collection. What is the expected size of this set?

Find the minimum possible value of

$$\sqrt{58-42 x}+\sqrt{149-140 \sqrt{1-x^{2}}}$$

where $-1 \leq x \leq 1$.

Five equally skilled tennis players named Allen, Bob, Catheryn, David, and Evan play in a round robin tournament, such that each pair of people play exactly once, and there are no ties. In each of the ten games, the two players both have a $50 \%$ chance of winning, and the results of the games are independent. Compute the probability that there exist four distinct players $P_{1}, P_{2}, P_{3}, P_{4}$ such that $P_{i}$ beats $P_{i+1}$ for $i=1,2,3,4$. (We denote $P_{5}=P_{1}$ ).

Two ordered pairs $(a, b)$ and $(c, d)$, where $a, b, c, d$ are real numbers, form a basis of the coordinate plane if $a d \neq b c$. Determine the number of ordered quadruples $(a, b, c, d)$ of integers between 1 and 3 inclusive for which $(a, b)$ and $(c, d)$ form a basis for the coordinate plane.

Horizontal parallel segments $A B=10$ and $C D=15$ are the bases of trapezoid $A B C D$. Circle $\gamma$ of radius 6 has center within the trapezoid and is tangent to sides $A B, B C$, and $D A$. If side $C D$ cuts out an arc of $\gamma$ measuring $120^{\circ}$, find the area of $A B C D$.

Emilia wishes to create a basic solution with $7 \%$ hydroxide $(\mathrm{OH})$ ions. She has three solutions of different bases available: $10 \%$ rubidium hydroxide $(\mathrm{Rb}(\mathrm{OH})), 8 \%$ cesium hydroxide $(\mathrm{Cs}(\mathrm{OH}))$, and $5 \%$ francium hydroxide $(\mathrm{Fr}(\mathrm{OH})$ ). ( $\mathrm{The} \mathrm{Rb}(\mathrm{OH})$ solution has both $10 \% \mathrm{Rb}$ ions and $10 \% \mathrm{OH}$ ions, and similar for the other solutions.) Since francium is highly radioactive, its concentration in the final solution should not exceed $2 \%$. What is the highest possible concentration of rubidium in her solution?

Mary has a sequence $m_{2}, m_{3}, m_{4}, \ldots$, such that for each $b \geq 2, m_{b}$ is the least positive integer $m$ for which none of the base- $b$ logarithms $\log _{b}(m), \log _{b}(m+1), \ldots, \log _{b}(m+2017)$ are integers. Find the largest number in her sequence.

Each of the integers $1,2, \ldots, 729$ is written in its base-3 representation without leading zeroes. The numbers are then joined together in that order to form a continuous string of digits: $12101112202122 \ldots$. . How many times in this string does the substring 012 appear?

Rthea, a distant planet, is home to creatures whose DNA consists of two (distinguishable) strands of bases with a fixed orientation. Each base is one of the letters H, M, N, T, and each strand consists of a sequence of five bases, thus forming five pairs. Due to the chemical properties of the bases, each pair must consist of distinct bases. Also, the bases H and M cannot appear next to each other on the same strand; the same is true for N and T . How many possible DNA sequences are there on Rthea?

On a blackboard a stranger writes the values of $s_{7}(n)^{2}$ for $n=0,1, \ldots, 7^{20}-1$, where $s_{7}(n)$ denotes the sum of digits of $n$ in base 7. Compute the average value of all the numbers on the board.

Undecillion years ago in a galaxy far, far away, there were four space stations in the three-dimensional space, each pair spaced 1 light year away from each other. Admiral Ackbar wanted to establish a base somewhere in space such that the sum of squares of the distances from the base to each of the stations does not exceed 15 square light years. (The sizes of the space stations and the base are negligible.) Determine the volume, in cubic light years, of the set of all possible locations for the Admiral's base.

New this year at HMNT: the exciting game of $R N G$ baseball! In RNG baseball, a team of infinitely many people play on a square field, with a base at each vertex; in particular, one of the bases is called the home base. Every turn, a new player stands at home base and chooses a number $n$ uniformly at random from $\{0,1,2,3,4\}$. Then, the following occurs:

• If $n>0$, then the player and everyone else currently on the field moves (counterclockwise) around the square by $n$ bases. However, if in doing so a player returns to or moves past the home base, he/she leaves the field immediately and the team scores one point.
• If $n=0$ (a strikeout), then the game ends immediately; the team does not score any more points.

What is the expected number of points that a given team will score in this game?

Denote $\phi=\frac{1+\sqrt{5}}{2}$ and consider the set of all finite binary strings without leading zeroes. Each string $S$ has a "base- $\phi$ " value $p(S)$. For example, $p(1101)=\phi^{3}+\phi^{2}+1$. For any positive integer $n$, let $f(n)$ be the number of such strings $S$ that satisfy $p(S)=\frac{\phi^{48 n}-1}{\phi^{48}-1}$. The sequence of fractions $\frac{f(n+1)}{f(n)}$ approaches a real number $c$ as $n$ goes to infinity. Determine the value of $c$.

A positive integer $k$ is called powerful if there are distinct positive integers $p, q, r, s, t$ such that $p^{2}, q^{3}, r^{5}, s^{7}, t^{11}$ all divide $k$. Find the smallest powerful integer.

How many sequences of integers $\left(a_{1}, \ldots, a_{7}\right)$ are there for which $-1 \leq a_{i} \leq 1$ for every $i$, and

$$a_{1} a_{2}+a_{2} a_{3}+a_{3} a_{4}+a_{4} a_{5}+a_{5} a_{6}+a_{6} a_{7}=4 ?$$

Michael writes down all the integers between 1 and $N$ inclusive on a piece of paper and discovers that exactly $40 \%$ of them have leftmost digit 1 . Given that $N>2017$, find the smallest possible value of $N$.

An equiangular hexagon has side lengths $1,1, a, 1,1, a$ in that order. Given that there exists a circle that intersects the hexagon at 12 distinct points, we have $M<a<N$ for some real numbers $M$ and $N$. Determine the minimum possible value of the ratio $\frac{N}{M}$.

Ashwin the frog is traveling on the $x y$-plane in a series of $2^{2017}-1$ steps, starting at the origin. At the $n^{\text {th }}$ step, if $n$ is odd, then Ashwin jumps one unit to the right. If $n$ is even, then Ashwin jumps $m$ units up, where $m$ is the greatest integer such that $2^{m}$ divides $n$. If Ashwin begins at the origin, what is the area of the polygon bounded by Ashwin's path, the line $x=2^{2016}$, and the $x$-axis?

Consider five-dimensional Cartesian space

$$\mathbb{R}^{5}=\left\{\left(x_{1}, x_{2}, x_{3}, x_{4}, x_{5}\right) \mid x_{i} \in \mathbb{R}\right\}$$

and consider the hyperplanes with the following equations:

• $x_{i}=x_{j}$ for every $1 \leq i<j \leq 5$;
• $x_{1}+x_{2}+x_{3}+x_{4}+x_{5}=-1 ;$
• $x_{1}+x_{2}+x_{3}+x_{4}+x_{5}=0$;
• $x_{1}+x_{2}+x_{3}+x_{4}+x_{5}=1$.

Into how many regions do these hyperplanes divide $\mathbb{R}^{5}$ ?

There are 12 students in a classroom; 6 of them are Democrats and 6 of them are Republicans. Every hour the students are randomly separated into four groups of three for political debates. If a group contains students from both parties, the minority in the group will change his/her political alignment to that of the majority at the end of the debate. What is the expected amount of time needed for all 12 students to have the same political alignment, in hours?

Find the number of quadruples $(a, b, c, d)$ of integers with absolute value at most 5 such that

$$\left(a^{2}+b^{2}+c^{2}+d^{2}\right)^{2}=(a+b+c+d)(a-b+c-d)\left((a-c)^{2}+(b-d)^{2}\right)$$

Let $A, B, C, D$ be points chosen on a circle, in that order. Line $B D$ is reflected over lines $A B$ and $D A$ to obtain lines $\ell_{1}$ and $\ell_{2}$ respectively. If lines $\ell_{1}, \ell_{2}$, and $A C$ meet at a common point and if $A B=4, B C=3, C D=2$, compute the length $D A$.

Yannick has a bicycle lock with a 4-digit passcode whose digits are between 0 and 9 inclusive. (Leading zeroes are allowed.) The dials on the lock is currently set at 0000 . To unlock the lock, every second he picks a contiguous set of dials, and increases or decreases all of them by one, until the dials are set to the passcode. For example, after the first second the dials could be set to 1100,0010 , or 9999, but not 0909 or 0190 . (The digits on each dial are cyclic, so increasing 9 gives 0 , and decreasing 0 gives 9.) Let the complexity of a passcode be the minimum number of seconds he needs to unlock the lock. What is the maximum possible complexity of a passcode, and how many passcodes have this maximum complexity? Express the two answers as an ordered pair.

Suppose $x$ is a rational number such that $x \sqrt{2}$ is also rational. Find $x$.

Regular octagon CHILDREN has area 1. Find the area of pentagon CHILD.

The length of a rectangle is three times its width. Given that its perimeter and area are both numerically equal to $k>0$, find $k$.

Alec wishes to construct a string of 6 letters using the letters A, C, G, and N, such that:

• The first three letters are pairwise distinct, and so are the last three letters;
• The first, second, fourth, and fifth letters are pairwise distinct.

In how many ways can he construct the string?

Define a sequence $\left\{a_{n}\right\}$ by $a_{1}=1$ and $a_{n}=\left(a_{n-1}\right)!+1$ for every $n>1$. Find the least $n$ for which $a_{n}>10^{10}$.

Lunasa, Merlin, and Lyrica each have a distinct hat. Every day, two of these three people, selected randomly, switch their hats. What is the probability that, after 2017 days, every person has their own hat back?

Compute

$$100^{2}+99^{2}-98^{2}-97^{2}+96^{2}+95^{2}-94^{2}-93^{2}+\ldots+4^{2}+3^{2}-2^{2}-1^{2}$$

Regular octagon CHILDREN has area 1. Determine the area of quadrilateral LINE.

A malfunctioning digital clock shows the time $9: 57 \mathrm{AM}$; however, the correct time is $10: 10 \mathrm{AM}$. There are two buttons on the clock, one of which increases the time displayed by 9 minutes, and another which decreases the time by 20 minutes. What is the minimum number of button presses necessary to correctly set the clock to the correct time?

Compute $\frac{x}{w}$ if $w \neq 0$ and $\frac{x+6 y-3 z}{-3 x+4 w}=\frac{-2 y+z}{x-w}=\frac{2}{3}$.

Find the sum of all real numbers $x$ for which

$$\lfloor\lfloor\cdots\lfloor\lfloor\lfloor x\rfloor+x\rfloor+x\rfloor \cdots\rfloor+x\rfloor=2017 \text { and }\{\{\cdots\{\{\{x\}+x\}+x\} \cdots\}+x\}=\frac{1}{2017}$$

where there are $2017 x$ 's in both equations. ( $\lfloor x\rfloor$ is the integer part of $x$, and $\{x\}$ is the fractional part of $x$.) Express your sum as a mixed number.

Trapezoid $A B C D$, with bases $A B$ and $C D$, has side lengths $A B=28, B C=13, C D=14$, and $D A=15$. Let diagonals $A C$ and $B D$ intersect at $P$, and let $E$ and $F$ be the midpoints of $A P$ and $B P$, respectively. Find the area of quadrilateral $C D E F$.

Fisica and Ritmo discovered a piece of Notalium shaped like a rectangular box, and wanted to find its volume. To do so, Fisica measured its three dimensions using a ruler with infinite precision, multiplied the results and rounded the product to the nearest cubic centimeter, getting a result of $V$ cubic centimeters. Ritmo, on the other hand, measured each dimension to the nearest centimeter and multiplied the rounded measurements, getting a result of 2017 cubic centimeters. Find the positive difference between the least and greatest possible positive values for $V$.

Points $A, B, C$, and $D$ lie on a line in that order such that $\frac{A B}{B C}=\frac{D A}{C D}$. If $A C=3$ and $B D=4$, find $A D$.

On a $3 \times 3$ chessboard, each square contains a knight with $\frac{1}{2}$ probability. What is the probability that there are two knights that can attack each other? (In chess, a knight can attack any piece which is two squares away from it in a particular direction and one square away in a perpendicular direction.)

A repunit is a positive integer, all of whose digits are 1 s. Let $a_{1}<a_{2}<a_{3}<\ldots$ be a list of all the positive integers that can be expressed as the sum of distinct repunits. Compute $a_{111}$.

A standard deck of 54 playing cards (with four cards of each of thirteen ranks, as well as two Jokers) is shuffled randomly. Cards are drawn one at a time until the first queen is reached. What is the probability that the next card is also a queen?

Mr. Taf takes his 12 students on a road trip. Since it takes two hours to walk from the school to the destination, he plans to use his car to expedite the journey. His car can take at most 4 students at a time, and travels 15 times as fast as traveling on foot. If they plan their trip optimally, what is the shortest amount of time it takes for them to all reach the destination, in minutes?

The skeletal structure of coronene, a hydrocarbon with the chemical formula $\mathrm{C}_{24} \mathrm{H}_{12}$, is shown below.

Each line segment between two atoms is at least a single bond. However, since each carbon (C) requires exactly four bonds connected to it and each hydrogen $(\mathrm{H})$ requires exactly one bond, some of the line segments are actually double bonds. How many arrangements of single/double bonds are there such that the above requirements are satisfied?

Rebecca has four resistors, each with resistance 1 ohm. Every minute, she chooses any two resistors with resistance of $a$ and $b$ ohms respectively, and combine them into one by one of the following methods:

• Connect them in series, which produces a resistor with resistance of $a+b$ ohms;
• Connect them in parallel, which produces a resistor with resistance of $\frac{a b}{a+b}$ ohms;
• Short-circuit one of the two resistors, which produces a resistor with resistance of either $a$ or $b$ ohms.

Suppose that after three minutes, Rebecca has a single resistor with resistance $R$ ohms. How many possible values are there for $R$ ?

A box contains three balls, each of a different color. Every minute, Randall randomly draws a ball from the box, notes its color, and then returns it to the box. Consider the following two conditions: (1) Some ball has been drawn at least three times (not necessarily consecutively). (2) Every ball has been drawn at least once.

What is the probability that condition (1) is met before condition (2)?

A sequence of positive integers $a_{1}, a_{2}, \ldots, a_{2017}$ has the property that for all integers $m$ where $1 \leq m \leq 2017,3\left(\sum_{i=1}^{m} a_{i}\right)^{2}=\sum_{i=1}^{m} a_{i}^{3}$. Compute $a_{1337}$.

A string of digits is defined to be similar to another string of digits if it can be obtained by reversing some contiguous substring of the original string. For example, the strings 101 and 110 are similar, but the strings 3443 and 4334 are not. (Note that a string is always similar to itself.) Consider the string of digits

$$S=01234567890123456789012345678901234567890123456789$$

consisting of the digits from 0 to 9 repeated five times. How many distinct strings are similar to $S$ ?

Triangle $A B C$ has side lengths $A B=15, B C=18, C A=20$. Extend $C A$ and $C B$ to points $D$ and $E$ respectively such that $D A=A B=B E$. Line $A B$ intersects the circumcircle of $C D E$ at $P$ and $Q$. Find the length of $P Q$.

Fisica and Ritmo discovered a piece of Notalium shaped like a rectangular box, and wanted to find its volume. To do so, Fisica measured its three dimensions using a ruler with infinite precision, multiplied the results and rounded the product to the nearest cubic centimeter, getting a result of 2017 cubic centimeters. Ritmo, on the other hand, measured each dimension to the nearest centimeter and multiplied the rounded measurements, getting a result of $V$ cubic centimeters. Find the positive difference between the least and greatest possible positive values for $V$.

Points $A, B, C, D$ lie on a circle in that order such that $\frac{A B}{B C}=\frac{D A}{C D}$. If $A C=3$ and $B D=B C=4$, find $A D$.

On a $3 \times 3$ chessboard, each square contains a Chinese knight with $\frac{1}{2}$ probability. What is the probability that there are two Chinese knights that can attack each other? (In Chinese chess, a Chinese knight can attack any piece which is two squares away from it in a particular direction and one square away in a perpendicular direction, under the condition that there is no other piece immediately adjacent to it in the first direction.)

Compute the number of functions $f:\{1,2, \ldots, 9\} \rightarrow\{1,2, \ldots, 9\}$ which satisfy $f(f(f(f(f(x)))))=$ $x$ for each $x \in\{1,2, \ldots, 9\}$.

Consider a sequence $x_{n}$ such that $x_{1}=x_{2}=1, x_{3}=\frac{2}{3}$. Suppose that $x_{n}=\frac{x_{n-1}^{2} x_{n-2}}{2 x_{n-2}^{2}-x_{n-1} x_{n-3}}$ for all $n \geq 4$. Find the least $n$ such that $x_{n} \leq \frac{1}{10^{6}}$.

Given complex number $z$, define sequence $z_{0}, z_{1}, z_{2}, \ldots$ as $z_{0}=z$ and $z_{n+1}=2 z_{n}^{2}+2 z_{n}$ for $n \geq 0$. Given that $z_{10}=2017$, find the minimum possible value of $|z|$.

In unit square $A B C D$, points $E, F, G$ are chosen on side $B C, C D, D A$ respectively such that $A E$ is perpendicular to $E F$ and $E F$ is perpendicular to $F G$. Given that $G A=\frac{404}{1331}$, find all possible values of the length of $B E$.

Let $P$ be a polynomial with integer coefficients such that $P(0)+P(90)=2018$. Find the least possible value for $|P(20)+P(70)|$.

Tetrahedron $A B C D$ with volume 1 is inscribed in circumsphere $\omega$ such that $A B=A C=A D=2$ and $B C \cdot C D \cdot D B=16$. Find the radius of $\omega$.

The skeletal structure of circumcircumcircumcoronene, a hydrocarbon with the chemical formula $\mathrm{C}_{150} \mathrm{H}_{30}$, is shown below.

Each line segment between two atoms is at least a single bond. However, since each carbon (C) requires exactly four bonds connected to it and each hydrogen $(\mathrm{H})$ requires exactly one bond, some of the line segments are actually double bonds. How many arrangements of single/double bonds are there such that the above requirements are satisfied? If the correct answer is $C$ and your answer is $A$, you get $\max \left(\left[30\left(1-\left|\log _{\log _{2} C} \frac{A}{C}\right|\right)\right], 0\right)$ points.

Rebecca has twenty-four resistors, each with resistance 1 ohm. Every minute, she chooses any two resistors with resistance of $a$ and $b$ ohms respectively, and combine them into one by one of the following methods:

• Connect them in series, which produces a resistor with resistance of $a+b$ ohms;
• Connect them in parallel, which produces a resistor with resistance of $\frac{a b}{a+b}$ ohms;
• Short-circuit one of the two resistors, which produces a resistor with resistance of either $a$ or $b$ ohms.

Suppose that after twenty-three minutes, Rebecca has a single resistor with resistance $R$ ohms. How many possible values are there for $R$ ? If the correct answer is $C$ and your answer is $A$, you get $\max \left(\left\lfloor 30\left(1-\left|\log _{\log _{2} C} \frac{A}{C}\right|\right)\right\rfloor, 0\right)$ points.

A box contains twelve balls, each of a different color. Every minute, Randall randomly draws a ball from the box, notes its color, and then returns it to the box. Consider the following two conditions: (1) Some ball has been drawn at least twelve times (not necessarily consecutively). (2) Every ball has been drawn at least once.

What is the probability that condition (1) is met before condition (2)? If the correct answer is $C$ and your answer is $A$, you get $\max \left(\left\lfloor 30\left(1-\frac{1}{2}\left|\log _{2} C-\log _{2} A\right|\right)\right\rfloor, 0\right)$ points.