一道SAT数学题.

题目

一道SAT数学题.
if 4 bags of flour with an average weight of 15 pounds are put onto a shelf with 45-pound bags of flour,what is the average weight,in pounds,of all 8bags on the shelf?
(A)5
(B)10
(C)15
(D)20
(E)25
答案是10,但我怎么算都不是10...不知道是我没看懂题还是.....

答案

题目应该是这样的.
4袋平均15磅的面粉和4袋5磅的面粉,放在一个架子上,8袋在一起的平均质量是多少?（4*15+4*5）/8=10 选B