GWD28-29

meckychen · 发表于 2010-1-26 12:51:30

Q28:

Each of the 30 boxes in a certain shipment with either 10 pounds or 20 pounds, and the average (arithmetic mean) weight of the boxes in the shipment is 18 pounds. If the average weight of the boxes in the shipment is to be reduced to 14 pounds by removing some of the 20-pound boxes, how many 20-pound boxes must be removed?

A. 4
B. 6

C. 10
D. 20
E. 24
Answer: D

Why not B

Thank you.

meckychen · 发表于 2010-1-27 11:23:48

Can anyone help me?

Libbyyao · 发表于 2010-1-27 11:39:32

LZ 列两个方程就OK了
A+B=30 和10A+20B/30=18 算出A=6 B=24
然后设需移除X个B 得：10X6+20（24-X）/ 30-X=14 解得X=20

meckychen · 发表于 2010-1-27 13:21:56

Thank you
My answer is 20(B-m)=18*30-14*30
So, B-m=6
So, answer is B
Where am I wrong?

meckychen · 发表于 2010-1-27 13:22:21

I see
thank you

GWD28-29

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