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IMSO 2004 Mathematics – Taiwan Second Round – Exploration Question 3

IMSO Maths · IMSO 2004 Mathematics – Taiwan Second Round – Exploration

Question 3

Two people play a game with marbles in a box. They alternate turns. Each person can take one or two marbles at a time. The loser is the person who takes the last marble.

On your turn, how many marbles will you take to be sure of winning if there are:

(a) 6 marbles left;

(b) 8 marbles left;

(c) 20 marbles left?

(d) Change the rule: there are 100 marbles and each player can take 1, 2, 3 or 4 marbles per turn. You play first. How many marbles will you take to be sure of winning?

Explain how your strategy responds to the other player's later moves.