Question

PLEASE ANSWER QUICKLY!!

Martha has a 4-digit combination lock on her briefcase and has forgotten the combination. Each of the four digits on her lock can read any digit from 0 through 9. If she
knows that the first digit is 6, and the second digit is prime, how many numbers must Martha try before the lock is sure to open? (Note: Neither "0" nor "1" are prime
numbers!)

Answer 1

224

Explanation:

1st Digit - 9 (1 way)

2nd Digit - 2, 3, 5, 7 (4 ways)

3rd Digit - (10-2) ways

4th Digit - (10-3) ways

Therefore, the number of ways are 1 × 4 × 8 × 7

= 224 ways