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# birds

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Director
Joined: 17 Oct 2005
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12 Feb 2006, 17:19
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a total of 30 percent of the birds included in a certain migration study were male. If some of the birds migrated during the study and 20 percent of the migrating birds were male, what was the ratio of the migration rate for the male birds to the migration rate for the female birds?

(migration rate for birds of a certain sex = no. of birds of that sex migrating/ total no. of birds of that sex)

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VP
Joined: 21 Sep 2003
Posts: 1057

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Location: USA

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12 Feb 2006, 18:07
Is it 7:12?

Assume
100 birds (30M & 70F)
x migrate.

0.2x = Male & 0.8x=Female

MM = Male migration ratio = 0.2x/30
FM = Female migration ratio = 0.8x/70

MM/FM = [0.2x/30]/[0.8x/70] = 7/12
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GMAT Club Legend
Joined: 07 Jul 2004
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Location: Singapore

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12 Feb 2006, 18:40
Assume total in study = y
Female = 0.7y
Male = 0.3y

x migrated during studyg -> 0.2x male, 0.8x female

migration rate for male birds = 0.2x/0.3y
migration rate for female birds = 0.8x/0.7y

Ratios male: female = 0.2x/0.3y * 0.7y/0.8x = 7/12

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Senior Manager
Joined: 11 Jan 2006
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Location: Chennai,India

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12 Feb 2006, 22:11
first go i got 1/7 , calculation mistake, agree it is 7/12
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Director
Joined: 17 Oct 2005
Posts: 924

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13 Feb 2006, 14:11
the oa is 7/12 great work guys

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13 Feb 2006, 14:11
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