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

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Intern
Joined: 21 Nov 2007
Posts: 11
If a coin has an equal probability of landing heads up or  [#permalink]

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27 Dec 2007, 01:43
3
9
00:00

Difficulty:

25% (medium)

Question Stats:

69% (01:10) correct 31% (01:15) wrong based on 708 sessions

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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
Manager
Joined: 26 Dec 2007
Posts: 70

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28 Dec 2007, 18:39
2
3
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
##### General Discussion
CEO
Joined: 17 Nov 2007
Posts: 3438
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
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27 Dec 2007, 03:12
2
1
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.
q - probability of tails.

P=3C2*(1/2)^2*(1/2)^1=3/8=0.375
Manager
Joined: 26 Feb 2007
Posts: 86

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29 Dec 2007, 06:40
2
probability of 2 heads =3C2 *1/2^3 =3/8 =0.375
Manager
Joined: 27 Oct 2008
Posts: 177

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27 Sep 2009, 10:34
3
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
Senior Manager
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Joined: 29 Sep 2010
Posts: 263
Concentration: General Management, Strategy
Schools: Ross '14 (M\$)

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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
Retired Moderator
Joined: 16 Nov 2010
Posts: 1419
Location: United States (IN)
Concentration: Strategy, Technology

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29 Mar 2011, 18:23
3C2 * (1/2)^2(1/2)

= 3 * 1/8

= 0.375
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Intern
Joined: 07 Aug 2013
Posts: 19
Re: If a coin has an equal probability of landing heads up or  [#permalink]

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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
Director
Joined: 10 Mar 2013
Posts: 503
Location: Germany
Concentration: Finance, Entrepreneurship
GMAT 1: 580 Q46 V24
GPA: 3.88
WE: Information Technology (Consulting)
If a coin has an equal probability of landing heads up or  [#permalink]

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19 Jan 2016, 12:42
Richard Lee wrote:
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

Probability * # of arrangements --> $$1/2^3*3=0,375$$
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Re: If a coin has an equal probability of landing heads up or  [#permalink]

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23 Sep 2018, 17:28
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Re: If a coin has an equal probability of landing heads up or &nbs [#permalink] 23 Sep 2018, 17:28
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