Advent calendar 2019
20 December
The integers from 2 to 14 (including 2 and 14) are written on 13 cards (one number per card). You and a friend take it in turns to take one of the numbers.
When you have both taken five numbers, you notice that the product of the numbers you have collected is equal to the product of the numbers that your friend has collected.
What is the product of the numbers on the three cards that neither of you has taken?
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Because 13 and 11 are prime and larger than 7, no-one can have taken them, as their is no way the other person's product could then be the same. Once 13 and 11 are discarded,
the prime factorisation of the product of all the other cards is \(2^{11}\times3^5\times5^2\times7^2\). In order to be shared to give the same product, the powers must be even, and so the other card not taken must be 6 (as 2 and 3 are the numbers with odd powers).
Therefore the product of the three cards is 858.
