[問題]天山7第14題

[michelle610]

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

答案是B 可是我不懂要怎麼算ㄟ

[freshmanduke]

按公式就可求出
(20/30) / (80/70) = 7/12

[michelle610]

多謝F大

[CUGGYER]

請問為什麼可以這樣算阿><
公式怎麼推導的呢 (我知道題目有附公式)

這題想好久都沒辦法理解耶....

[SugarChunda]

令total number of geese為 X, total number of the mugrating geese為Y, 則可以得到 :
the migration rate for the male geese = 20%Y / 30%X .... (1)
the migration rate for the female geese = 80%Y / 70%X .... (2)

(1) / (2) 即為所求