ANSWER
10,000
EXPLANATION
Given:
The first-three digits to be 1, 8, 0 out of seven-digit telephone numbers.
Desired outcome:
Total number of possible different telephone numbers
Determine the possibilities of the 4th, 5th, 6th and the 7th digits
The 4th digit has 10 possibilities (i.e: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
The 5th also has 10 possibilities,
likewise the 6th and the 7th digits.
Now, we have:
10 x 10 x 10 x 10 = 10^4 = 10,000
Hence, the number of possible different telephone numbers is 10,000.
