The internal telephone numbers in the phone system on a campus consists of 4 digits, with the 1st not equal to 0. How many different numbers can be assigned in this system?

This system have 9000 different numbers.

Step-by-step explanation:

We know that the internal telephone numbers in the phone system on a campus consists of 4 digits, with the 1st not equal to 0.

We have total 10 digits.

In the first place we have 9 possibilities, because from the conditions of the task in the first place there cannot be 0.

In second, third and fourth place we have 10 possibilities.

Therefore, we get

This system have 9000 different numbers.