Problem Solving
试题详情
题目：
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?
选项:
A、4
B、6
C、10
D、20
E、24
答案:
D
提问：
这道题我错选B，解题思路是：平均值从18降到14，就应该是18*30-14*30=120（减掉的总重量），再用120除以20得到6，请问这样的思路哪里错了呢
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