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Probablity - A certain telephone number has 7 digits.

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by Magellan » Thu May 15, 2008 3:57 am
We have 4 digits (7 minus 3 fixed) that can be potentially prime.

Those digit can take the following values: 2, 3, 4, 5, 6, 7, 8, 9
Out of those digits, half are prime: 2, 3, 5, 7

To get to the result, we will use:
Probability(At least 1 prime) = 1 - Probability(No Prime at all)

We know:
Probability no prime for 1st digit: 1/2
Probability no prime for 2nd digit: 1/2
Probability no prime for 3rd digit: 1/2
Probability no prime for 4th digit: 1/2

Therefore, probability(No prime at all) = 1/2 * 1/2 * 1/2 * 1/2 = 1/16

And we get
Probability(At least 1 prime) = 1 - Probability(No Prime at all) = 1 - 1/16 = 15/16
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by sandeep_chhabra » Thu May 15, 2008 4:03 am
Oops !! it was my bad :(

i did the same but finally forgot that i have got the probability of No Primes and need to subtract it from 1.

Thanks!
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