Answer:
3024
Step-by-step explanation:
You have to think of this in choices. You have 9 choices for the first lock (because there are 9 numbers). We would have 9 choices for the second lock, but we used one number for the previous one. So we would have 8 choices. We would continue with 7 choices for the third lock and 6 for the fourth.
Multiplying this we get 9x8x7x6=3024
