求助陈向东的一道题?

Each of the interger from 0 to 9, inclusive, is written on a separate slip of blank paper and the ten slips are

dropped into a hat.

If the slips are then drawn one at a time without replacement,

how many must be drawn to ensure that the numbers on two of the slips drawn will have a sum of 10?

answer is 7.

why 7 times?

0-9 不重复向外取，问至少取多少次，取出的号码其中的2个之和是10？

最坏的情况，从0开始拿，0-5，这6个卡片没有相加得10的，剩下6-9，随便拿一个就行了。

所以是7个

请问你的陈向东的数学资料是哪下的？可否给链接啊？谢谢！

google邮箱

gmatoncd(在) gmail.com
    

密码:123321gmat