Arandomly generated password contains four characters. each of the four characters is either a lowercase letter or a digit from 0–9. each character in the password cannot be used more than once.
quickly
what is the approximate probability that exactly one of the four characters will be a number?

1%
11%
28%
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%