Answer:
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
[tex]N=9\cdot 10\cdot10 \cdot 10=9000[/tex]
This system have 9000 different numbers.
