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Probability ( Really difficult)

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by akpareek » Fri Jul 30, 2010 5:18 am
If a certain coin is flipped, the probability that the coin will land heads is 1/2. If the coin
is flipped 5 times, what is the probability that it will land heads up on the first 3 flips and
not on the last 2 flips?
A. 3/5
B. 1/2
C. 1/5
D. 1/8
E. 1/32
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by selango » Fri Jul 30, 2010 5:41 am
P(H)=1/2

P(T)=1-1/2=1/2[Prob of not getting Heads]

P(H) on first flip=1/2

P(H) on second flip=1/2

P(H) on third flip=1/2

P(T) on fourth flip=1/2

P(T) on fifth flip=1/2

=(1/2)^5=1/32

Pick E
--Anand--
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by Rahul@gurome » Fri Jul 30, 2010 5:46 am
akpareek wrote:If a certain coin is flipped, the probability that the coin will land heads is 1/2. If the coin
is flipped 5 times, what is the probability that it will land heads up on the first 3 flips and
not on the last 2 flips?
A. 3/5
B. 1/2
C. 1/5
D. 1/8
E. 1/32
Probability = Number of ways an event can occur/ Total number of possible outcomes
Total number of possible outcomes when the coin is flipped 5 times = 2^5 = 32
Number of ways so that we get heads up on the first 3 flips and not on the last 2 flips is only 1 (HHHTT)
So, the required probability = 1/32

The correct answer is [spoiler](E)[/spoiler].
Rahul Lakhani
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by Gurpinder » Fri Jul 30, 2010 8:06 am
not that this approach is the most efficient way, but it works.....

you can actually draw a probability tree. I got the same answer as the other 2 people who responded - 1/32.

and simply count the number of outcomes you want over the total number of outcomes there are.

again....its not the most efficient way but it WORKS when you are stuck and don't know what to do.
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