Question 1143232
first 3 digits are 3 5 0.


1. how many telephone numbers may be assigned to that city if the last 4 digits can't begin or end with 0?


total number of possibilities is 1 * 1 * 1 * 9 * 10 * 10 * 9 = 8,100.


the first 3 digits are fixed.
the fourth digit has 9 possibilities (1 to 9).
the fifth and sixth digits each have 10 possibilities (0 to 9).
the seventh digit has 9 possibilities (1 to 9).


2. how many telephone numbers may be assigned to that city if the last 4 digits have no restrictions?


total number of possibilities is 1 * 1 * 1 * 10 * 10 * 10 * 10 = 10,000.


the first 3 digits are fixed.
the last 4 digits each have 10 possibilities (0 to 9).


3. how many telephone numbers may be assigned to that city if there is no repetition of digits?


total number of possibilities is 1 * 1 * 1 * 7 * 6 * 5 * 4 = 840.


the first 3 digits are fixed.
the fourth digit has 7 possibilities (0 to 9 except any of the numbers chosen for the first 3 digits.
the fifth digit has 6 possibilities (0 to 9 except any of the numbers chosen for the first 4 digits.
the sixth digit has 5 possibilities (0 to 9 except any of the numbers chosen for the first 5 digits.
the seventh digit has 4 possibilities (0 to 9 except any of the numbers chosen for the first 6 digits.


that's what i think.
give it a shot and see if it works for you.