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AMC 12 2015b Problem 20

  • Paper

For every positive integer nn, let mod5(n)\text{mod}_5 (n) be the remainder obtained when nn is divided by 5. Define a function f:{0,1,2,3,… }×{0,1,2,3,4}→{0,1,2,3,4}f: \{0,1,2,3,\dots\} \times \{0,1,2,3,4\} \to \{0,1,2,3,4\} recursively as follows:

f(i,j)={mod5(j+1) if i=0 and 0≤j≤4,f(i−1,1) if i≥1 and j=0, andf(i−1,f(i,j−1)) if i≥1 and 1≤j≤4.f(i,j) = \begin{cases}\text{mod}_5 (j+1) & \text{ if } i = 0 \text{ and } 0 \le j \le 4 \text{,}\\ f(i-1,1) & \text{ if } i \ge 1 \text{ and } j = 0 \text{, and} \\ f(i-1, f(i,j-1)) & \text{ if } i \ge 1 \text{ and } 1 \le j \le 4. \end{cases}

What is f(2015,2)f(2015,2)?

\textbf{(A)} 0 \textbf{(B)} 1 \textbf{(C)} 2 \textbf{(D)} 3 \textbf{(E)} 4

Dataset text AMC 12 2000 to 2025. Quoted with source.

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B

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