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# If the probability of rain on any given day in City X is 50

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Intern
Joined: 25 Sep 2007
Posts: 22
If the probability of rain on any given day in City X is 50  [#permalink]

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04 Jan 2008, 16:01
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15% (low)

Question Stats:

79% (01:32) correct 21% (01:59) wrong based on 503 sessions

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If the probability of rain on any given day in City X is 50 percent, what is the probability that it rains on exactly 3 days in a 5-day period?

(A) 8/125
(B) 2/25
(C) 5/16
(D) 8/25
(E) 3/4
Math Expert
Joined: 02 Sep 2009
Posts: 49860
Re: If the probability of rain on any given day in City X is 50  [#permalink]

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27 Feb 2012, 03:40
6
7
flokki wrote:
If the probability of rain on any given day in City X is 50 percent, what is the probability that it rains on exactly 3 days in a 5-day period?

(A) 8/125
(B) 2/25
(C) 5/16
(D) 8/25
(E) 3/4

Could you tell me why it is a combination and not a permutation?

N = 3C5 x 1/8 x 1/4

why do i have to divide by 2! ?

The probability of rain each day is 1/2 and the probability of no rain is also 1/2. $$C^3_5=10$$ represent ways to choose on which 3 days out of 5 there will be a rain, so $$P=C^3_5*(\frac{1}{2})^3*(\frac{1}{2})^2=\frac{10}{32}=\frac{5}{16}$$.

Or think about it this way: we want the probability of the following event: RRRNN, where R represent rain day and N represents no-rain day. Now, each R and each N have individual probability of 1/2, so $$(\frac{1}{2})^5$$.

But the case of RRRNN can occur in many ways: RRRNN, RRNRRN, RNRRN, NRRRN, ... basically it will be equla to # of arrangements (permutations) of 5 letters RRRNN out of which there are 3 identical R's and 2 identical N's. That # of arrangements is $$\frac{5!}{3!2!}$$, (notice that it's the same as $$C^3_5$$). So, finally $$P=\frac{5!}{3!2!}*(\frac{1}{2})^5=\frac{5}{16}$$.

For more on this topic check Combinations and Probability chapters of Math Book:
math-combinatorics-87345.html
math-probability-87244.html

Also check similar questions to practice:
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Hope it helps.
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27 Sep 2009, 11:02
5
2
If the probability of rain on any given day in City X is 50 percent, what is the probability that it rains on exactly 3 days in a 5-day period?

(A) 8/125
(B) 2/25
(C) 5/16
(D) 8/25
(E) 3/4

Soln:
Let Y represent raining on a day and N represent Not raining on a day
So for it to rain 3 out of 5 days we have YYYNN.
Since the order in which it rains in 5 days matters hence we find the total number of possibilities
But since Y has a happened 3 times and N has happened 2 times we cannot distinguish between them and hence divide by the repetitions . Thus we need to divide by (3! * 2!)
=> 5!/(2! * 3!)
=> 10 ways

Now since on each day it can rain or it cannot rain. Thus total outcomes is
=> 2 * 2 * 2 * 2 * 2
=> 32

So probability that it rains on exactly 3 days is
= 10/32
= 5/16

Ans is C
##### General Discussion
Director
Joined: 08 Jun 2007
Posts: 553

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04 Jan 2008, 16:11
1
4
JuliaS wrote:
If the probability of rain on any given day in City X is 50 percent, what is the probability that it rains on exactly 3 days in a 5-day period?

(A) 8/125
(B) 2/25
(C) 5/16
(D) 8/25
(E) 3/4

Thanks a lot!

welcome to the forum

total possibilties 2^5 = 32
rain combinations 5c3 = 10
10/32 = 5/16
Director
Joined: 01 May 2007
Posts: 776

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04 Jan 2008, 17:55
Why is total possibilties 2^5 = 32?
Intern
Joined: 30 Dec 2007
Posts: 10

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04 Jan 2008, 19:19
vscid wrote:
yeah how did u arrive at 2^5.

Let me try to elaborate
Each day, we have 2 possibilities, rain or not rain, therefore the total possibility of 5 days is 2^5=32
Then, we pick 3 rainy days randomly from 5 days, 5C3 = 10
10/32=5/16
Manager
Joined: 22 Jul 2009
Posts: 169

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18 Sep 2009, 18:45
5
Binomial distribution formula: C(n,k) * p^k * (1-p)^(n-k)

Given that the probability of Rain happening is p (=1/2) and not happening is 1-p (=1-1/2=1/2),
=> Probability of Rain happening k times (=3) in n repeated tests (=5)
= C(5,3) * (1/2)^3 * (1-1/2)^(5-3)
= C(5,3) * (1/2)^5
= 10/32
= 5/16
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Posts: 217
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Concentration: General Management, Entrepreneurship
GMAT 1: 750 Q49 V44
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03 Aug 2011, 19:10
Hi,

I'm confused by the answers, only one answer actually has the 50% probability factored in. How come all the other answers just seemed to mention how many ways to choose 3 days in 5 days?

Seems like 50% is a special case, because when you do 5c3, the 3 days could be 3 days of rain or even 3 days of no rain. They all end up being the same probability because of the 50%.

how would this be solved with 20%? 80%?
Manager
Joined: 06 Feb 2011
Posts: 62
WE: Information Technology (Computer Software)

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05 Aug 2011, 13:45
pinchharmonic wrote:
Hi,

I'm confused by the answers, only one answer actually has the 50% probability factored in. How come all the other answers just seemed to mention how many ways to choose 3 days in 5 days?

Seems like 50% is a special case, because when you do 5c3, the 3 days could be 3 days of rain or even 3 days of no rain. They all end up being the same probability because of the 50%.

how would this be solved with 20%? 80%?

Eg: For the 20% possibility of rain it will be (1/5)^3 * (4/5)^2* (5C3)
Intern
Joined: 01 Dec 2011
Posts: 2
Re: If the probability of rain on any given day in City X is 50  [#permalink]

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27 Feb 2012, 03:15
Could you tell me why it is a combination and not a permutation?

N = 3C5 x 1/8 x 1/4

why do i have to divide by 2! ?
Intern
Joined: 01 Dec 2011
Posts: 2
Re: If the probability of rain on any given day in City X is 50  [#permalink]

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27 Feb 2012, 03:58

I just started with permutations and probability.

I still dont understand why we use the combination formula and not the permutation one.

Is it because we do not differentiate between one rainy day and the other? are they all the same for us?

Are most Gmat questions at this level?
Math Expert
Joined: 02 Sep 2009
Posts: 49860
Re: If the probability of rain on any given day in City X is 50  [#permalink]

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27 Feb 2012, 04:10
flokki wrote:

I just started with permutations and probability.

I still dont understand why we use the combination formula and not the permutation one.

Is it because we do not differentiate between one rainy day and the other? are they all the same for us?

Are most Gmat questions at this level?

Yes, we do not differentiate between RRR and NN: for example RRRNN means that it was raining on the first three days and we have no reason to differentiate between them. Also notice that most GMAT combination/probability questions are fairly straightforward and can be solved in several ways. This problem is also dealing with a simple concept explained in the links I sited above, so I'd recommend to brush your fundamentals on combinations before you move to the problems.
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Re: If the probability of rain on any given day in City X is 50  [#permalink]

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29 Feb 2012, 11:26
Binomial formula b(x; n, p) = nCx p^x (1-p)^ n-x
n=3; m=5; p=1/2; 1-p=1/2
b(3; 5, 1/2) = 5C3 * 1/2^2 * 1/2^5
= 10 * 1/32
= 10/32
= 5/16
Manager
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Schools: IE '15 (A)
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If the probability of rain on any given day in City X is 50  [#permalink]

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07 Jan 2013, 12:42
select three rainy days out of 5 = 5C3 = 10
probability of 3 rainy and 2 non rainy days = (1/2)^5

Answer = 10 * 1/32 = 5/16
Manager
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Re: Probability it with rain on 3 days in a five day ?  [#permalink]

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07 Jan 2013, 15:01
probabiliy of raining on 3 days (1/2)^3 x (1/2)^2 = 1/32

the total number of arragements of rain on 3 days and no rain on 2 days is 5!/2!3! = 10

therefore, probability of raining on exactly 3 days during a 5 day week is 10/32 = 5/16
Intern
Joined: 14 Jan 2018
Posts: 1
Re: If the probability of rain on any given day in City X is 50  [#permalink]

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12 Feb 2018, 13:37
how can we solve this kind of problem with different percentages of probability?

So instead of 50 percent lets say 30 percent, I know how to solve this in the "Binomial formula" way, but what about the "n/N" way (N=total possibilities n=rain combinations) what are the total possibilities in the case of 30 percent chances of rain?

thank you
Re: If the probability of rain on any given day in City X is 50 &nbs [#permalink] 12 Feb 2018, 13:37
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