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 我選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 又題目說只要把20 pound的箱數移掉使得平均重量為14 pounds, 假設平均重量14 pound 的20-pound 有 r 箱 所以, 得到: 60 + 20r = 420, r= 360/20 = 18 也就是說20 pound 的箱數有18箱, 所以 24-18=6 箱 就是我求到的答案了 錯了之後,我又算了ㄧ次, 這一次我決定要把未知數r 設成"需要移動的箱數" 60+20*(24-r)=14(6+24-r) 所以 60 + 480 + 20r = 420 + 14r 6r = 120 ---> r = 20 (冏") 這真是太神奇了...但是我還是不明白我第一次錯的原因在哪裡 有人知道嗎??? --

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