问一道GMAT数学题!
问题描述：
问一道GMAT数学题!
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 randomly from the box,without returning the cards he had already drawn to the box.In order to ensure that the sum of all te cards he drew was even,how many card did Jerome have to draw?
答
Jerome在每张卡片上写下1-20号,将这些卡片放在盒子里面.每次从盒子里面无放回任意拿出一张卡片.为了保证他拿出的卡片上面数字之和为偶数,他必须取多少张?
假设第一张是偶数,接下来取的都是10张偶数,那么每取一张,它们的和始终都是奇数
只有等10张偶数全部取完,接下来再取就是奇数,它们的和就是偶数
所以：必须取1+10+1=12张第一张是奇数吧？对对对，是我写错了！