Sarah cannot completely remember her four-digit ATM pin number. She does remember the first two digits, and she knows that each
of the last two digits is greater than 5. The ATM will allow her three tries before it blocks further access. If she randomly guesses the last two digits, what is the probability she will get access to her account
Since she remembers the first two digits, she only has to guess the last two digits. If both digits are greater than 5, there are 4 possible alternatives for each digit (6, 7, 8 or 9).
In three tries, the probability that she will get it right is:
The probability she will get access to her account is 3/16 or 0.1875.
