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?
第一张是奇数吧?
追答对对对,是我写错了!
可是为什么答案是12啊?是不是题目错了?我也觉得是20
追答12?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?