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A mother bird has 6 babies. Everytime she returns to the nest,she feed

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A mother bird has 6 babies. Everytime she returns to the nest,she feed [#permalink]

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New post 14 Sep 2010, 11:27
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Question Stats:

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A mother bird has 6 babies. Everytime she returns to the nest, she feeds half the babies, even if some of those babies were fed the last time around. If it takes her 5 minutes to feed each baby, how long will it take her to feed all the possible combinations of three babies?

A 5 hours
B 6 hours
C 8 hours
D 2 hours
E 12 hours

[Reveal] Spoiler:
ANS A
bird can feed 3 out of 6 so,combinations=
6C3=20
5 minutes to feed each baby so 15 minutes to feed a combination....=15*20=300minutes.
=5hours

Can someone please explain me the explanation...

Procedure i followed is
as there are 3 bird combinations,order doesnt matter and also the repititons are allowed-
the formula is (n+r-1)!/r!(n-1)! which gives a different ans

please help me with this problem....
[Reveal] Spoiler: OA

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Re: A mother bird has 6 babies. Everytime she returns to the nest,she feed [#permalink]

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New post 14 Sep 2010, 11:36
Not sure where you get the formula from.

The number of ways to pick r out n objects where order does not matter is :

\(C(n,r)=\frac{n!}{r!*(n-r)!}\)

This is how you get 20.
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New post 14 Sep 2010, 11:40
http://www.mathsisfun.com/combinatorics ... tions.html

this is the URl which gives that formula

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New post 14 Sep 2010, 11:41
Order doesn't matter and repetetion is not allowed so 6*5*4/3*2*1 = C(6,3) = 20 combincations.

I feed bird 1 or bird 2 first it does't matter. I cannot feed bird 1 if its already fed
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New post 14 Sep 2010, 11:44
saxenashobhit ,

",even if some of those babies were fed the last time around"
what does that mean..im really sorry for the silly question,but im not able to understand it...please explain...

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New post 14 Sep 2010, 11:46
harithakishore wrote:
http://www.mathsisfun.com/combinatorics/combinations-permutations.html

this is the URl which gives that formula


this is not a combination with repetition, this is just a regular combination. Each set of 3 is fed exactly once => No repetition
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New post 14 Sep 2010, 14:29
harithakishore wrote:
saxenashobhit ,

",even if some of those babies were fed the last time around"
what does that mean..im really sorry for the silly question,but im not able to understand it...please explain...

It says everytime mother bird goes to nest to feed the babies, last turns choice don't impact current turn. Which makes each turn independent.

Conditional problems need to be handled differently.
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New post 15 Sep 2010, 01:23
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in 6C3 ways we can select 3 birds out of 6 birds. Note that selecting a bird in multiple 3 member group os allowed as her mother can feed it multiple times.

Hence 6C3 = 20 groups, ecah group with 3 members,
Hence total 20*3 = 60 birds selected --> means one bird is selected for 10 times.

5 mins to feed one bird --> 60*5 mins to feed 60 birds (i.e. 20 groups) = 300mins = 5 hours

ANSWER: A

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Re: A mother bird has 6 babies. Everytime she returns to the nest,she feed [#permalink]

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Re: A mother bird has 6 babies. Everytime she returns to the nest,she feed   [#permalink] 30 Sep 2017, 21:02
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