[求助]一道比例题. GWD1-33

xiaoyue

Q33:

At a certain school, the ratio of the number of second graders to the number of fourth graders is 8 to 5, and the ratio of the number of first graders to the number of second graders is 3 to 4. If the ratio of the number of third graders to the number of fourth graders is 3 to 2, what is the ratio of the number of first graders to the number of third graders?

A. 16 to 15

B. 9 to 5

C. 5 to 16

D. 5 to 4

E. 4 to 5

以前都没有这个讨论题..可能太简单.但偶还是迷惑..

求解法.

himba

答案选E.偶的做法给楼主看看,一起讨论.

用数字1g,2g,3g,4g分别代表4个年级.那把条件一个一个列出来:

2g/4g=8/5 (1)

1g/2g=3/4 (2)

3g/4g=3/2 (3)

求1g/3g

观察上面的比例式子,要得到1g/3g,可以用(2)/(3)得到

1g/2g*4g/3g=1g*4g/2g*3g=1g/3g*4g/2g=1/2

由(1) 2g/4g=8/5可知道4g/2g=5/8

那么1g/3g*5/8=1/2

1g/3g=4/5

*是乘法,担心楼主看不太清楚.不知道这样解释是否清楚哦

所以选Ｄ