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Re: The probability of rain on each of five days is 1/6, except on the fir [#permalink]
Bunuel wrote:
The probability of rain on each of five days is 1/6, except on the first day, when it is 2/5, and on the last day, when it is 4/5. What is the probability that rain will occur on at least one of the five days?

(A) 1/675
(B) 5/72
(C) 5/27
(D) 22/27
(E) 67/72


Probability of rain on at least one of the days = 1 - (Probability of rain on no day).

Probability of rain on Day-1 = \(\frac{2}{5}\), no rain = \(\frac{3}{5}\).
Probability of rain on Day-2 = Day-3 = Day-4 = \(\frac{1}{6}\), no rain = \(\frac{5}{6}\).
Probability of rain on Day-4 = \(\frac{4}{5}\), no rain = \(\frac{1}{5}\).

Probability of rain on at least one of the days = \(1 - (\frac{3}{5} * (\frac{5}{6})^{3} * \frac{1}{5}) = 1 - \frac{15}{216} = 1 - \frac{5}{72} = \frac{67}{72}\). Ans - E.
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Re: The probability of rain on each of five days is 1/6, except on the fir [#permalink]
Probability that rain will not happen on any day is-

=(1-2/5)(1-1/6)(1-1/6)(1-1/6)(1-4/5)
=3/5*5/6*5/6*5/6*1/5
=5/72

now for probability of rain to occur on at least one of five days, We will subtract probability of not occurring into 1 i.e

=1-5/72
=67/72

Answer is E
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Re: The probability of rain on each of five days is 1/6, except on the fir [#permalink]
What if it was asked to find the probability of raining for atleast 2 days??

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Re: The probability of rain on each of five days is 1/6, except on the fir [#permalink]
Expert Reply
Bunuel wrote:
The probability of rain on each of five days is 1/6, except on the first day, when it is 2/5, and on the last day, when it is 4/5. What is the probability that rain will occur on at least one of the five days?

(A) 1/675
(B) 5/72
(C) 5/27
(D) 22/27
(E) 67/72


Solution:

The probability that it will rain on at least one of the five days is 1 minus the probability that it will not rain on any of the five days. The probability that it will not rain on any of the five days is 3/5 x 5/6 x 5/6 x 5/6 x 1/5 = (3 x 5)/(6 x 6 x 6) = 5/(2 x 6 x 6) = 5/72. Therefore, the probability that it will rain on at least one of the five days is 1 - 5/72 = 67/72

Answer: E
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Re: The probability of rain on each of five days is 1/6, except on the fir [#permalink]
Expert Reply
Probability of at least 1-day rain: 1 - (No rain on any day)

Probability of rain on first day : \(\frac{2 }{ 5}\) ; hence probability of no rain on first day : 1 - \(\frac{2 }{ 5}\) = \(\frac{3 }{ 5}\)

Probability of rain on second, third and fourth day (each) : \(\frac{1 }{ 6}\) ; hence probability of no rain on each day : 1 - \(\frac{1 }{ 6}\) = \(\frac{5 }{ 6}\)


Probability of rain on last day : \(\frac{4 }{ 5}\) ; hence probability of no rain on last day : 1 - \(\frac{4 }{ 5}\) = \(\frac{1 }{ 5}\)

Overall Probability of at least 1-day rain: 1 -( \(\frac{3 }{ 5}\)*\(\frac{5 }{ 6}\)*\(\frac{5 }{ 6}\)*\(\frac{5 }{ 6}\)*\(\frac{1 }{ 5}\))

=> 1 - \(\frac{5 }{ 72}\)

=> \(\frac{67 }{ 72}\)

Answer E
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Re: The probability of rain on each of five days is 1/6, except on the fir [#permalink]
Bunuel wrote:
The probability of rain on each of five days is 1/6, except on the first day, when it is 2/5, and on the last day, when it is 4/5. What is the probability that rain will occur on at least one of the five days?

(A) 1/675
(B) 5/72
(C) 5/27
(D) 22/27
(E) 67/72

Can someone explain concept behind- at least - questions..also is there any post where I can get all probability concepts and formulas..thanks in advance.

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Re: The probability of rain on each of five days is 1/6, except on the fir [#permalink]
shreyasaxena2301 wrote:
Bunuel wrote:
The probability of rain on each of five days is 1/6, except on the first day, when it is 2/5, and on the last day, when it is 4/5. What is the probability that rain will occur on at least one of the five days?

(A) 1/675
(B) 5/72
(C) 5/27
(D) 22/27
(E) 67/72

Can someone explain concept behind- at least - questions..also is there any post where I can get all probability concepts and formulas..thanks in advance.

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Find here - https://gmatclub.com/forum/math-probability-87244.html
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Re: The probability of rain on each of five days is 1/6, except on the fir [#permalink]
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