Problem Solving

My answer is 1/16 but QA is 15/16... i dont understand why it should be 15/16

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

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!