MATH PREP TEST 2 NO 133 求助

what is the average (arithmetic mean) height of the n people in a certain group?

(1) the average height of the n/3 tallest people n the group is 6 feet 2.5 inches ,and the average height of the rest of the people n the group is f feet 10 inches

(2) the sum of the heights of the n people is 178 feet 9 inches.

the right answer given is A .

But i consider D is best .

Could you give me some hints?

我模考也选的D

后来想想

1 可以把N 消掉

2  178 feet 9 inches/N

第二个式子已经知道了所有高度的和,再除以N , 不就可以得出平均身高了吗?

那为什么二式不成立?

求教

知道总数 关键是你并不知道n是多少 不就求不出平均数了吗 178 feet 9 inches/N=?

奥，我把N 看成是已知数了。谢谢楼上