Question

laura has a 3-digit combination lock on her briefcase and has forgtten the combination. If she knows that the first digit is a 3, and the second digit is a prime, how many numbers must Laura try before the lock is sure to open?

Answer 1

Laura needs to try 36 more times before the lock is sure to open.

Explanation:

Alright, with a three digit lock, there are 9 possible numbers for each digit:

1, 2, 3, 4, 5, 6, 7, 8, and 9.

Since the first one is 3, lets move onto the second digit, which is prime.

Of the numbers between 1-9,

2, 3, 5, and 7 are prime.

With the last digit, it can be any number 1-9.

With all of this, there are 4 combos left for the second digit then 9 combos for the third digit. Therefore, there would be 36 possible combinations as 4 * 9 =36.