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# This one has been done before : A coin is flipped 5

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Manager
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This one has been done before : A coin is flipped 5 [#permalink]

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02 Aug 2003, 17:15
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This one has been done before :

A coin is flipped 5 times. What is the probability that there are at least 3 heads ?

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Eternal Intern
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03 Aug 2003, 09:01
2^5 = 32 outcome

2C1 + 2C1 + 2C1
------------------
32

am I right
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03 Aug 2003, 10:54
10C3+5C1+1C1
---------------------=16/32=1/2
32

Or

at least 3 means 3, 4, and 5 heads
the opposite cases are 0, 1, and 2 heads
the rule of symmetry gives 3/6=1/2

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03 Aug 2003, 11:24
coin is flipped 5 times. What is the probability that there are at least 3 heads ?

Stolyar, do you realize 10C3 works out to 120, I'm lost here.

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03 Aug 2003, 13:04
Can you not use the Binomial Equation ?

P = 5C3*(1/2)^3*(1/2)^2

Last edited by tzolkin on 03 Aug 2003, 16:04, edited 1 time in total.

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03 Aug 2003, 13:51
Although I've never heard of it until now, Tzolkin, if you state we should spend more time doing "ETS" style, then don't worry about this question. Spend more time with algebra and geometry, why are we all obsessed about probability
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04 Aug 2003, 23:28
I agree that Binomial method can be applied to this problem

p1 = probability that it is Heads = 1/2
p2 = probablity that it is not Heads = 1/2

P = 5C3 (p1)^3 (p2)^2 = 0.3215

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05 Aug 2003, 00:29
tzolkin wrote:
Can you not use the Binomial Equation ?

P = 5C3*(1/2)^3*(1/2)^2

Tzolkin

The case you state here is only for 3 heads. The question asks for ATLEAST three heads = 3 or 4 or 5 heads.

Therefore, using bionomial equation

P(atleast 3 heads) = 5C3+5C4+5C5 / (1/2)^5 = 16/32 = 1/2

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05 Aug 2003, 00:29
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# This one has been done before : A coin is flipped 5

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