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Animal population

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Animal population [#permalink]  12 Mar 2011, 11:29
In a certain animal population, for each of the first 3 months of life, the probability that an animal will die during that month is 1/10. For a group of 200 newborn members of the population, approximately how many would be expected to survive the first 3 months of life?
A/ 140
B/ 146
C/ 152
D/ 162
E/ 170
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Re: Animal population [#permalink]  12 Mar 2011, 11:39
200/10=20 will die in the first month.
Left 180
180/10=18 will die in the 2nd
Left 162
162/10=16.2=16 will die in the 3rd

Total dead = 20+18+16 = 54
Alive = 200-54= 146

Ans: "B"
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Re: Animal population [#permalink]  12 Mar 2011, 11:41
Expert's post
In a certain animal population, for each of the first 3 months of life, the probability that an animal will die during that month is 1/10. For a group of 200 newborn members of the population, approximately how many would be expected to survive the first 3 months of life?
A/ 140
B/ 146
C/ 152
D/ 162
E/ 170

The probability of survival for each of the first 3 months of life is 1-1/10=9/10, so of 200 newborn 200*9/10*9/10*9/10=~146 is expected to survive.

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Re: Animal population [#permalink]  12 Mar 2011, 11:48
Good question.

Its important to see the given death probability is for one month, otherwise one can get this wrong.
Re: Animal population   [#permalink] 12 Mar 2011, 11:48
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