大家帮我看看这道题

nicktao

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

我算出是B　　是不是KEY错了

heatostrich

答案是对的。你是不是粗心了~

设10磅的有x个

10X+20*(30-X)=18*30

x=6

设第二种情况中20磅的有Y个

10*6+20Y=14（6+Y）

Y=4

30-6-4=20

nicktao

果然是我算错了。。

哎。。这种低级状态怎么保证49－51呀