one probability question

[Dkumar]

Hi Brinda!
I get a different answer for your question.

On any day in April in a certain city, the probability of rain is 50%. What is the probability of that it rains exactly 5 days during a 7 day period in April?

Acc to my calculation,

Probability that it will rain in a any day is 1/2
Prabability that it will not rain is (1-1/2) = 1/2
Probability that it will rain exactly 5 days and will not rain the other two days = 1/2 *1/2*1/2*1/2*1/2*1/2*1/2 = 1/128

Please let me know the correct answer.
Thanks.

[manasi4gre]

The probability is 1/2 that a certain coin will turn up heads on any given toss. If the coin is to be tossed three times, what is the probability that on at least one of the tosses the coin will turn up heads?
The answer is 3/8.

How did u get this answer?

[ish]

Hi Brinda!
I get a different answer for your question.

On any day in April in a certain city, the probability of rain is 50%. What is the probability of that it rains exactly 5 days during a 7 day period in April?

Acc to my calculation,

Probability that it will rain in a any day is 1/2
Prabability that it will not rain is (1-1/2) = 1/2
Probability that it will rain exactly 5 days and will not rain the other two days = 1/2 *1/2*1/2*1/2*1/2*1/2*1/2 = 1/128

Please let me know the correct answer.
Thanks.

Kumar,you have to use the binomial method here.
nCr(p)^r(q)^n-r
so here its
7C2(50/100)5(50/100)2
which is
21(1/2)^7

[manasi4gre]

ish, why cant we use the other approach?

[ish]

because you have to account for the number of ways you can choose 5 out of 7, and then compute their respective probabilities. its also a question of choice, isnt it?

[manasi4gre]

yes I know that.. was actually wondering about the other approach..anyway, the solution is clear now.

[Dkumar]

Yeah you are right. I have to work more in this area.
Thanks a lot ish. Keep posting more problems brinda.

[brindha_arun]

thanks dinesh for ur encouragement. i will try to post as many problems as i can

Yeah you are right. I have to work more in this area.
Thanks a lot ish. Keep posting more problems brinda.

[vit11]

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