gwd 8 33 比例問題

Kath1985 · 发表于 2011-6-15 22:43:02

Q33:
Ateachergavethesametesttothreehistoryclasses:A, B, and C. The average (arithmetic mean) scores for thethree classes were 65, 80, and 77, respectively. The ratio of the numbers of students in each class who took thetest was 4 to 6 to 5, respectively. What was the average score for thethree classes combined?

A. 74
B. 75
C. 76
D. 77
E. 78
是B
前人說這樣解:
65*4+80*6+77*5=260+480+385=1125
1125/(4+6+5)=75
但平均=總數除以總個數 4 5 6 是比例並非個數 怎麼可以用比例代替正確的個數值求答案呢?
例如總值=80個數比=2:3 80除以(2+3)=16
但事實上個數確切值為20、30 80除以(20+30)=1.8 兩者求出的答案根本不同這也是我說的總個數要是確切的值不可用總個數的比例去算平均值呀

gwd 8 33 比例問題

所属分类: GMAT考试

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