Question 387985
For the first problem you will just have to count the number of prime numbers between 1 and 100. (Remember that 1 is not considered prime or composite.) The probability will be the number of primes over the total number of possible numbers (100).<br>
For the second problem, since it does not say that the 4 letters must be difference we must assume that repetitions are possible. And we must also assume that your friend gets only one guess.<br>
The number of possible passwords would be:
26*26*26*26*10*10*10
The probability of your friend guessing your password in one guess:
1/26*26*26*26*10*10*10
(Use your calculator to simplify the denominator.)