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HMMT 2019 C10

  • Combinatorics
  • Answer

Fred the Four-Dimensional Fluffy Sheep is walking in 4-dimensional space. He starts at the origin. Each minute, he walks from his current position (a1,a2,a3,a4)\left(a_{1}, a_{2}, a_{3}, a_{4}\right) to some position (x1,x2,x3,x4)\left(x_{1}, x_{2}, x_{3}, x_{4}\right) with integer coordinates satisfying (x1−a1)2+(x2−a2)2+(x3−a3)2+(x4−a4)2=4\left(x_{1}-a_{1}\right)^{2}+\left(x_{2}-a_{2}\right)^{2}+\left(x_{3}-a_{3}\right)^{2}+\left(x_{4}-a_{4}\right)^{2}=4 and ∣(x1+x2+x3+x4)−(a1+a2+a3+a4)∣=2\left|\left(x_{1}+x_{2}+x_{3}+x_{4}\right)-\left(a_{1}+a_{2}+a_{3}+a_{4}\right)\right|=2. In how many ways can Fred reach (10,10,10,10)(10,10,10,10) after exactly 40 minutes, if he is allowed to pass through this point during his walk?

Dataset text NuminaMath olympiads. Quoted with source.

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\binom{40}{10}\binom{40}{20}^{3}

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