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 Ans: D 我选B(= ="), and here are reasons why: Let there are x 10-pound boxes and y 20-pounds boxes and thus (1) x + y = 30 and (2) 10x + 20y = 540 --->x + 2y = 54 Therefore, y = 24 and x = 6 又题目说只要把20 pound的箱数移掉使得平均重量为14 pounds, 假设平均重量14 pound 的20-pound 有 r 箱 所以, 得到: 60 + 20r = 420, r= 360/20 = 18 也就是说20 pound 的箱数有18箱, 所以 24-18=6 箱 就是我求到的答案了 错了之後,我又算了ㄧ次, 这一次我决定要把未知数r 设成"需要移动的箱数" 60+20*(24-r)=14(6+24-r) 所以 60 + 480 + 20r = 420 + 14r 6r = 120 ---> r = 20 (冏") 这真是太神奇了...但是我还是不明白我第一次错的原因在哪里 有人知道吗??? --

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