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PROBABILITY question

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by Mclaughlin » Tue Jun 03, 2008 8:57 pm
If the probability of rain on any given day in City X is 50%, 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
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by netigen » Tue Jun 03, 2008 9:08 pm
ans = (1/2)^3(1/2)^2 5C3

= (1/32) x 5x2 = 5/16

is OA C
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by Mclaughlin » Tue Jun 03, 2008 9:18 pm
Can you explain how you figured it out, are you using some sort of formula? I dont know why you broke it down in fractions like that.

THANKS IN ADVANCE.
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by netigen » Tue Jun 03, 2008 9:27 pm
Yes there is one. Read up on this tutorial and Perm/Comb and probability will be much easier for you

https://www.mansw.nsw.edu.au/members/ref ... no4yen.htm
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by Mclaughlin » Tue Jun 03, 2008 9:43 pm
Sadly, i've read that already a few times I am just stupid and can never figure out which formula i need to use when I get to a problem. Grrr. Thanks for your help.
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by zacharyz » Wed Jun 04, 2008 1:32 am
How did you know to multiply by the combination in this case? What wording gave you that, beyond the 1/(2^5) calculation...
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by molt_llest » Wed Jun 04, 2008 2:13 am
I did this,

I know that of 5 days has to rain 3--> so we have the combination (5 3)= 10
Also I know that in every days there is a chance of 50% to rain, so 50%*50%*50%*... =0,5^5

If you multiple 10*0.5^5 you'll get 5/16.
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by nareshchellani » Wed Jun 04, 2008 5:24 am
50% Chance means probability of rain in any day is 1/2

and 1/2 no rain.

Q's has ask rain exactly 3 days so 1/2*1/2*1/2 and do not rain for 2 days 1/2*1/2, now selection of 3 days from 5 days 5C3.

I hope u understood. Now a twist , if Q has been ask at least 3 day rain then.....
3 Day + 4 Day + 5 Day
1/2^3*1/2^2*5C3 + 1/2^4*1/2*5C4 + 1/2^5 * 5C5.
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