Question

A randomly generated password has four characters. Each character is either A, B, C, D, E or F or a number from 0 -

9. Each character in the password can only be used once. What is the probability that a password has only one
number?

Answer 1

44%

Sample space is 36C4

Now, we want to know all of the combinations that have 1 digit in it.

So, we can have one here:

1XXX

X1XX

XX1X

XXX1

But we have 10 different digits to choose from. So, we need to introduce the combination term, nCr, where n is a list of all digits and r is how many we want.

Since we only want one, we will need 10C1 for the number of digits. But we need to choose three lowercases, so it becomes 10C1 × 26C3

Since it's a probability question, we need to divide that by our sample space, 36C4, and our percentage becomes 44%