问两道数学题

LUCKY潘潘 · 发表于 2010-6-11 22:42:14

A square countertop has a square tile inlay in the center , leaving an untiled strip of uniform width around the tile. If the ratio of the tiled area to the untiled area is 25 to 39,which of the following could be the width ,in inches , of the strip?
答案是1,3,4都行。怎么做的啊？
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 on at a timerandomly 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 he haxw to draw?答案是12。怎么得出的啊？
请大家帮忙想想，不胜感激！

秋儿叶 · 发表于 2010-6-11 23:10:54

第二道题假设很神奇的取出的都是偶数在你没有彻底排除有几个奇偶时
那么你取了10个后偶数米了
那么再取2个奇数就能保证是even
所以要取12个

问两道数学题

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