A telephone number has seven digits, no number starts with 0. In a city, how many different telephone numbers the formed using the digits 0 to 6?
(Each digit can occur only once)

Options

A. 6!
B. 6.6!
C. 7!
D. 2.7!

Explanation

(i) The first digit of the number cannot be zero, so it can filled in 6 ways.
(ii) Now the second digit can be any of the 6 digits, i.e., it can be filled in 6 ways.
(iii) As the digits cannot be repeated, the number of ways the third digit can be filled is 5 ways and so on.
(iv) Now apply the fundamental theoram.