JBMO-SL 2019 C5
An economist and a statistician play a game on a calculator which does only one operation. The calculator displays only positive integers and it is used in the following way: Denote by an integer that is shown on the calculator. A person types an integer, , chosen from the set of the first 99 positive integers, and if of the number is again a positive integer, then the calculator displays of . Otherwise, the calculator shows an error message and this operation is not allowed. The game consists of doing alternatively these operations and the player that cannot do the operation looses. How many numbers from guarantee the winning strategy for the statistician, who plays second?
For example, if the calculator displays 1200, the economist can type 50 , giving the number 600 on the calculator, then the statistician can type 25 giving the number 150 . Now, for instance, the economist cannot type 75 as of 150 is not a positive integer, but can choose 40 and the game continues until one of them cannot type an allowed number.
Dataset text NuminaMath olympiads. Quoted with source.
Solutions
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