求助，数学题

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)]

我选B

在某移民鹅研究中,所有鹅的30%是公的. 如果在研究中,其中一些鹅移民了,且移了民的鹅的20%是公的.问公鹅移民率比母鹅移民率是多少? [那个性别的移民率=那个性别移民鹅的数目/那个性别的鹅的总数]

假设共有鹅G只,移民了共M只

那么可知,共有公鹅30%G只,母鹅70%G只. 移民了的公鹅有20%M只,移民了的母鹅有80%M只.

所以求的比率应该=(20%M/30%G) / (80%M/70%G)=7/12

谢谢rotty。