Add 2 or Subtract 10

You and a friend play a game where you start with a pile of 1000 stones, and each turn you can either add 2 stones to the pile or remove 10 stones. So, once you have fewer than 10 stones in the pile, you can only add 2 stones.

The player to remove the last stone wins.

Which player has a winning strategy?


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The one who goes second Neither player The one who goes first

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2 solutions

Geoff Pilling
Jun 22, 2017

Every two turns the number of stones in the pile will be divisible by four. Therefore, the first player will never be able to remove the last stone, since he will always be presented with a number that is divisible by 4.

The strategy for the second player is to keep removing 10 stones until the number of stones left is less than 10.

Then, when the number gets to 10, remove the final 10!

I didn't realize you couldn't remove all the stones if there were less than 10 :( Could you maybe clarify that?

Alex Li - 3 years, 10 months ago

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Done. How's that?

Geoff Pilling - 3 years, 10 months ago

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Much clearer, thanks

Alex Li - 3 years, 10 months ago
Jaydee Lucero
Jul 6, 2017

Let person 1 be the one who plays first, and person 2 be the one who plays second. Let A A be the number of "add 2 stones" moves by persons 1 and 2 combined, and S S be the number of "subtract 10 stones" moves by persons 1 and 2 combined.

The event "no stone is left" satisfies the equation 1000 + 2 A 10 S = 0 , 1000+2A-10S=0, or rewriting, A = 5 S 500. A=5S-500. Clearly, A 0 A \geqslant 0 , and thus S 100 S\geqslant 100 . Now, if S S is even, then S = 2 k , k Z + S=2k,k\in\mathbb{Z^+} , and A = 5 ( 2 k ) 500 = 10 k 500 = 10 ( k 50 ) 0 ( m o d 2 ) A=5(2k)-500=10k-500=10(k-50)\equiv 0 \pmod{2} so that A A is also even. Now, if S S is odd, so that S = 2 k + 1 , k Z + S=2k+1, k\in\mathbb{Z^+} , then A = 5 ( 2 k + 1 ) 500 = 10 k 495 = 5 ( 2 k 99 ) 1 ( m o d 2 ) A=5(2k+1)-500=10k-495 = 5(2k-99) \equiv 1 \pmod{2} so that A A is also odd.

In both cases, we have shown that A + S A+S is always even. But A + S A+S is the total number of moves done by persons 1 and 2 combined. Therefore, this implies that if person 1 moves first, then the game ends with person 2 as the last move (the winning move). Thus the one who goes second has the winning strategy.

PS: Actually, it follows from above that even if person 2 doesn't have any strategy, person 2 will always win no matter what moves they do! (provided, of course, that person 1 moves first)

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