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Birds

by MBA.Aspirant » Sat Jun 18, 2011 12:20 pm
At the moment there are 54,210 tagged birds in a certain wildlife refuge. If exactly 20 percent of all birds in the refuge are tagged, what percent of the untagged birds must be tagged so that half of all birds in the refuge are tagged?


25
30
33 1/3
37 1/2
50
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by pemdas » Sat Jun 18, 2011 12:36 pm
it's 37 1/2%

untagged birds are 80%
half of all birds are 50%
tagged birds are 20%

(50-20)/80=30/80=3/8
MBA.Aspirant wrote:At the moment there are 54,210 tagged birds in a certain wildlife refuge. If exactly 20 percent of all birds in the refuge are tagged, what percent of the untagged birds must be tagged so that half of all birds in the refuge are tagged?


25
30
33 1/3
37 1/2
50
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by MBA.Aspirant » Sat Jun 18, 2011 12:44 pm
pemdas wrote:
(50-20)/80=30/80=3/8
Explain the reasoning behind it pls
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by Brent@GMATPrepNow » Sat Jun 18, 2011 1:39 pm
MBA.Aspirant wrote:
pemdas wrote:
(50-20)/80=30/80=3/8
Explain the reasoning behind it pls
pemdas is essentially saying that, since the question is asking for a percent, we don't need to consider the actual numbers of birds.

In fact, if we're going to use numbers, let's say there are 100 birds.

This means that 20 are tagged and 80 are untagged.

For 1/2 the birds to be tagged, we need 50 tagged birds. In other words, we need 30 additional tagged birds.

So, from the 80 untagged, we need to tag 30 birds.

30/80 = 3/8 = 37.5%

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