14.

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?

lovetiky, hopefully u could give me an explanation.

假设样本里面有100只, 30只公的.走了20只,其中4只公的.

那么剩下80只里26只公的, 结果就是26: 54=>13:27

"total" * 30% = "male"

"migrating" * 20% = "male migrating"

suppose that "migrating" = "total" * a%

migration rate for male geese is ("male migrating")/("male")

= ("migrating" * 20%) / ("total" * 30%)

= ("total" * a% * 20%) / ("total" * 30%)

=(a% * 20%)/(30%)

similiarly, migration rate for female geese is ("female migrating")/("female")

= ("migrating" * (1 - 20%)) / ("total" * (1 - 30%))

= ("total" * a% * 80%) / ("total" * 70%)

=(a% * 80%)/(70%)

so the ratio is ((a% * 20%)/(30%)) / ((a% * 80%)/(70%)) = 7/12

哎呀,俺做错了.sorry ,看题不仔细

应该是(4只跑了的男鹅/原来的30只男鹅) /(16只跑了的女鹅/原有的70只女鹅)= (4/30)/(16/70)=7/12

[此贴子已经被作者于2006-8-3 10:00:32编辑过]