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gmat prep2 - coin toss

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Senior | Next Rank: 100 Posts
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by gkammaje » Sun Feb 15, 2009 6:17 pm
OA - E

ok..i suck at probabilities and my wild guess was wrong... is this considered a hard question or an easy one?! :?: :?
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by DeepakR » Sun Feb 15, 2009 6:48 pm
Probability will be heads atleast 4 times when coin is tossed 5 times =P

P=(4H and 1T) + P(all 5 times it is H)

We know that from question, P(H)=0.6 and P(T)=0.4
P(4H and 1T)= - - - - - there are 5 places from which you need to select 1 T and the rest will be H. So 5C1*4C4=5 arrangements like
THHHH,HTHHH,HHTHH,HHHTH,HHHHT = 5 possibilities.

Hence P(4H and 1T) = 5*(0.6)^4 * 0.4 and P(all 5 times H)=0.6^5

Total P= 5*(0.6)^4*(0.4) + (0.6)^5 =Ans.) E

- Deepak
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