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 o
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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 o
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?
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 o
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
我觉得是20吧= =要不总会有一种组合使得sum为odd可是为什么答案是12啊？是不是题目错了？我也觉得是2012？如果标着12345679的八个没有取出，那么和显然是奇数啊。。。而且如果是随机取的话，箱子外和是奇数的期望应该一直是百分之50，直到最后一张取出来。。。不懂= =是我理解错意思了么？...
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我觉得是20吧= =要不总会有一种组合使得sum为odd
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答案应该为20
x=my,(x+2)=ny (m,n均是自然数）
my=ny-2
(m-n)y=2
2只有1与2这两个因数，所以
m-n=1,y=2或
m-n=2,y=1
因为y≠1，所以只能是y=2