gretchen knows the combination to a bank vault is two letters followed by 2 numbers.
A) what is the probability that gretchen guesses the combination on her frist attempt
B) suppose she knows the combination starts with the letter M
what is the probability she guesses the comination on her frist attempt.
A) Allowing for repeated letters/numbers we have (26)^2 * (10)^2 = 67,600 possible codes
So....the chance of guessing correctly the first time is 1 /67,600 ≈ 0.0000148 = .00148%
B) With M being the first letter, there are 26 * 10^2 codes = 2600 possible codes
The probability of a correct guess on the first attempt = 1/2600 ≈ 0.0003846 = .03846 %