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85) Brad flips a two-sided coin 8 times. What is the

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85) Brad flips a two-sided coin 8 times. What is the [#permalink]  19 Jul 2008, 21:13
85) Brad flips a two-sided coin 8 times. What is the probability that he gets tails on at least 7 of the 8 flips?

1/32

1/16

1/8

7/8

none of the above

the ans i get is 1/2^7, which is E. is this correct.
prob that he get atleast 7 out of 8 heads is

7 times head and 1 time tail= 1/2*1/2*1/2... 8 times ( since the prob of getting head and tail is same).
and
so 1/2^8+ 1/2^8= 2/2^8 = 1/2^7.
is this ok?
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Re: A gmat prep. is this correct? [#permalink]  19 Jul 2008, 21:51
We have 2 events.

1st: 7 Tails and 1 Head (8 Cases, HTTTTTTT,THTTTTTT,...,TTTTTTTH)
2nd: 8 Tails (1 Case, TTTTTTTT)

Possible cases = 8+1 = 9
Total cases = 2^8

Probability = 9/2^8 = 9/256

Even I get E but my calculations are different from yours.
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Re: A gmat prep. is this correct? [#permalink]  20 Jul 2008, 07:35
What is the probability of getting only one H in all those tries? 8
T T T T T T T H
T T T T T T H T
....
...
H T T T T T T T

T T T T T T T T

8+1/2^8 = 8/256 = 9/256
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Re: A gmat prep. is this correct? [#permalink]  20 Jul 2008, 07:54
arjtry..

there is an error in ur calculations.

you have taken the probability of 7 tails and 1 head as 1/(2^8). This is correct for any 1 particular position of head. But here the 1 head can occur in any of the 8 positions. So you should have taken 8* 1/(2^8).

plus all tails has the probability of 1/(2^8)

so probability of 7T,1H or 8T,0H is (8+1)/(2^8) = 9/256
Re: A gmat prep. is this correct?   [#permalink] 20 Jul 2008, 07:54
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