Question

How many $6$-digit telephone numbers can be constructed using the digits $0$ to $9$ if each number starts with $67$ and no digit appears more than once?

Answer 1

The third number can be made out of $8$ digits

$4^{th}$ number can be made out of $7$ digits

$5^{th}$ number can be made out of $6$ digits

Hence no of telephone numbers that can be formed are $8\times 7\times 6=336$ ways