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Coprime picking game on the integers 2 to 30

Question

Two players, Alice and Bob, alternately pick integers from {2,3,…,30}\{2,3,\dots,30\}. Whoever first picks a number having a common factor greater than 11 with some number already chosen loses. Alice may decide whether she moves first or second, and both play perfectly. Put p=1p=1 if Alice should move first and p=2p=2 if she should move second. It turns out Alice has exactly six winning opening picks v1,…,v6v_1,\dots,v_6. Compute 100p+v1+⋯+v66100p+\dfrac{v_1+\cdots+v_6}{6}.

Short answer

115. Alice should start (p=1p=1). Picking 66 (or 12,18,2412,18,24) kills primes 22 and 33; after that every remaining safe number uses exactly one of the 88 primes 5,…,295,\dots,29, so moves just knock out one prime each and Bob runs out first. Opening with 55 or 2525 also wins (confirmed by exhaustive search), giving 100+6+12+18+24+5+256=115100+\tfrac{6+12+18+24+5+25}{6}=115.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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