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HMMT 2018 A7

  • Algebra and Number Theory
  • Answer

Rachel has the number 1000 in her hands. When she puts the number xx in her left pocket, the number changes to x+1x+1. When she puts the number xx in her right pocket, the number changes to x−1x^{-1}. Each minute, she flips a fair coin. If it lands heads, she puts the number into her left pocket, and if it lands tails, she puts it into her right pocket. She then takes the new number out of her pocket. If the expected value of the number in Rachel's hands after eight minutes is EE, then compute ⌊E10⌋\left\lfloor\frac{E}{10}\right\rfloor.

Dataset text NuminaMath olympiads. Quoted with source.

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