GWD 1-19

[cuteone]

Each of the 30 boxes in a certain shipment weighs either 10 pounds or 20 pounds, and 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

我算是B....WHY是D呢??

[homaru]

設平均18 pounds時, 10 pounds的有x箱,20 pounds的有(30-x)箱
[10x+20(30-x)]/30=18
得出x=6-->20 pounds有24箱

設平均14 pounds時,10 pounds的有6箱,20 pounds的有y箱
(10*6+20*y)/(6+y)=14
得出y=4

24-4=20

[cuteone]

先謝謝這位大大了 (Y)

但是降算為何錯呢?
---> 平均14 pounds時, 總重量少了4*30=120pounds
然後120pounds 去除以 20......等於6

PS..小妹不才,需要指點

[homaru]

題目只說要減少20磅的箱子
沒有說要增加10磅的箱子
所以總數已經不是30箱嚕

[cuteone]

喔喔...原來如此
會錯意了

[dennis_wtliu]

提供另一個算法
設Z為被移除後,剩下的箱子總數
540-14Z = 20(30-Z)

可得Z= 10
30 - 10= 20 答案D