请教一题数学题

hupan · 发表于 2013-12-31 14:42:10

Jerome wrote each of the integers 1 through 20, inclusive, on a separate index card. He placed the cards in a box, then drew cards one at a time randomely from the box, without returning the cards he had already drawn to the box. In order to ensure that the sum of all the cards he drew was even, how many cards did Jerome have to draw?
A19
B12
C11
D10
E3
答案是12，就弄不懂这题什么意思？
12张的话取了7张偶数的5张奇数的那么和不还是奇数吗？

hupan · 发表于 2013-12-31 22:30:09

up一下求NN们给屌丝讲讲啊

hupan · 发表于 2014-1-3 00:14:46

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请教一题数学题

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