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If Jesse flips a coin seven times in a row, what is the probability th

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If Jesse flips a coin seven times in a row, what is the probability th [#permalink]

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If Jesse flips a coin seven times in a row, what is the probability that the result will be heads at least five times?

A. 21/128
B. 29/128
C. 35/128
D. 1/16
E. 1/4
[Reveal] Spoiler: OA

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Re: If Jesse flips a coin seven times in a row, what is the probability th [#permalink]

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New post 08 Jan 2016, 19:33
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shasadou wrote:
If Jesse flips a coin seven times in a row, what is the probability that the result will be heads at least five times?

A. 21/128
B. 29/128
C. 35/128
D. 1/16
E. 1/4


Hi,
since the Q asks us atleast 5 times, we will have to find 5 times, 6 times and 7 times..
1) 5 times..
there are 7C5 ways that the heads can be placed in 7 times..
the prob of heads or tails is 1/2..
so prob in each way of head=(1/2)^5*(1/2)^2..
total ways=7C5*(1/2)^5*(1/2)^2= 21*(1/2)^7..
2)6 times
7C6*(1/2)^6*(1/2)^1=7*(1/2)^7.
3)7 times
7C7*(1/2)^7*(1/2)^0=1*(1/2)^7...

total ways..
(21+7+1)*(1/2)^7=29/128
B
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shasadou wrote:
If Jesse flips a coin seven times in a row, what is the probability that the result will be heads at least five times?

A. 21/128
B. 29/128
C. 35/128
D. 1/16
E. 1/4




Hi,
I have explained the way to get this formula, but you can use this ai almost all 'atleast times' probability..
This is also called binomial prob formula..
prob of doing a thing x times out of total n ways= \(nCx*p^x\) *(1 − p)^n−x ,
where n denotes the number of trials and p denotes the success
probability.
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Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
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Re: If Jesse flips a coin seven times in a row, what is the probability th [#permalink]

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New post 08 Jan 2016, 20:13
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Question : Atleast 5 heads in 7 flip.


The total outcome of flip is = 2^7 = 128
For any Coins problem write the ask in the shown format.

HHHHHTT
HHHHHHT
HHHHHHH

Once you have written in the above mentioned format the answer is pretty straight.

HHHHHTT = [7!]/5!*2! = 21
HHHHHHT = [7!]/6! = 7
HHHHHHH = [7!]/[7!] = 1

Sum = 21+7+1 = 29/128

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Re: If Jesse flips a coin seven times in a row, what is the probability th [#permalink]

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Total outcome is \(2^7=128\)

Desired outcomes: \(7C5 + 7C6 +7C7 = 21 + 7 + 1\)

\(= \frac{29}{128}\)
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Re: If Jesse flips a coin seven times in a row, what is the probability th [#permalink]

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New post 09 Oct 2016, 10:20
shasadou wrote:
If Jesse flips a coin seven times in a row, what is the probability that the result will be heads at least five times?

A. 21/128
B. 29/128
C. 35/128
D. 1/16
E. 1/4


what a tricky damn one...
since order doesn't matter...i started with the number of ways we can get 5, 6, or even 7 heads...
5 heads = 7C5 = 21
6 heads = 7C6 = 7
7 heads = 7C7 = 1

total: 21+7+1 = 29.
this number, multiply by (1/2)^7.
1/2^7 = 1/128.
29/128 is the answer.

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Re: If Jesse flips a coin seven times in a row, what is the probability th [#permalink]

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New post 26 Oct 2017, 05:35
Probability = favorable outcomes / total outcomes

Favourable outcomes = no of times we get heads AT LEAST 5 times = no of times we get heads 5 times + no of times we get heads 6 times + no of times we get heads 7 times = 7C5 + 7C6 + 7C7 = 29

Total favorable outcomes = 2^7

So ans = 29/128

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Re: If Jesse flips a coin seven times in a row, what is the probability th   [#permalink] 26 Oct 2017, 05:35
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