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If a coin has an equal probability of landing heads up or

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If a coin has an equal probability of landing heads up or [#permalink] New post 27 Dec 2007, 01:43
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If a coin has an equal probability of landing heads up or tails up each time it is flipped , what is the probability that the coin will land heads up exectly twice in 3 consecutive flips ?

A. 0.125
B. 0.25
C. 0.375
D. 0.5
E. 0.666
[Reveal] Spoiler: OA
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 [#permalink] New post 27 Dec 2007, 03:12
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C

P=nCm*p^m*q^(n-m)
n - the total number of trials.
m - the number of trials with heads.
n-m - the number of trials with tails.
p - probability of heads.
q - probability of tails.

P=3C2*(1/2)^2*(1/2)^1=3/8=0.375
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 [#permalink] New post 28 Dec 2007, 18:39
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this also works....

we know that we want 2 H and 1 T, so we have 3 possibilities

HHT
HTH
THH

find the probability of each and then add....
HHT= 1/2*1/2*1/2 = 1/8
HTH= 1/2*1/2*1/2 = 1/8
THH= 1/2*1/2*1/2 = 1/8

1/8+1/8+1/8= 3/8 or 0.375
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 [#permalink] New post 29 Dec 2007, 06:40
probability of 2 heads =3C2 *1/2^3 =3/8 =0.375
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Re: coin probability [#permalink] New post 27 Sep 2009, 10:34
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If a coin has an equal probability of landing heads up or tails up each time it is flipped , what is the probability that the coin will land heads up exectly twice in 3 consecutive flips ?

A 0.125
B 0.25
C 0.375
D 0.5
E 0.666

Soln:
Total number of ways in which H or T can appear in 3 tosses of coin is
= 2 * 2 * 2 = 8 ways

For 2 H and 1 T
HHT, HTH, THH

Thus probability is
= P(HHT) + P(HTH) + P(THH)
= 1/8 + 1/8 + 1/8
= 3/8
= .375

Ans is C
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Re: coin probability [#permalink] New post 29 Mar 2011, 07:56
It actually helps reading and understanding the problem correctly (:
I understood the question as 'what is the probability that the coin will land heads up consequently twice in 3 consecutive flips'?

In that case:
HHT = 1/8
THH = 1/8
Total outcomes: 8
Probability: 1/4
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Re: coin probability [#permalink] New post 29 Mar 2011, 18:23
3C2 * (1/2)^2(1/2)

= 3 * 1/8

= 0.375
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Re: If a coin has an equal probability of landing heads up or [#permalink] New post 20 Sep 2013, 02:28
You can either determine that there are three outcomes (HTT, THT, TTH) that fit the criteria of use the formula 3C2 (3!/2!(3!-2!)

each outcome has a probality of 1/2*1/2*1/2 = 1/8

3 outocomes with P=1/8 = 3/8

1/8=0.125 or 3/8 is 0.375
Re: If a coin has an equal probability of landing heads up or   [#permalink] 20 Sep 2013, 02:28
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