请教GWD一数学题

haoqian2009

A total of 30 percent of the geese included in a certain migration study were male.  If some of the geese migrated during the study and 20 percent of the migrating geese were male, what was the ratio of the migration rate for the male geese to the migration rate for the female geese?

[Migration rate for geese of a certain sex = (number of geese of that sex migrating) / (total number of geese of that sex)]

 

A.    1/4

B.     7/12

C.     2/3

D.    7/8

E.     8/7

答案B

多谢！！

sqsouare

1、设原来总人数100，方便求解，如果觉得特殊了设成总人数A也可。

2、如果按上面的设法，原来有男生30人，女生70人。

3、migrated的男生占20%,则设这部分男生为x，则migrated女生为4x。

4、根据给出的公式，最后问题问的是：（x/30）/(4x/70)=7/12，打完收工。