题目

A large number of people arrange themselves into groups of 2,6or 10 people.The mean size of a group is 5. However, when each person is asked howmany other people are in their group(excluding themselves), the mean of their answers is 7. Prove that there are no groups of 6 people. 一大群人自行分为 2 人组、6 人组或 10 人组，每组的平均人数为5。但是当每一个人被问到自己所在组里还有多少人(除自己外)时，他们回答的人数的平均值是7。请证明其中没有6 人组。 ——、、、