How many 5-digit telephone numbers can be constucted using the digits 0 to 9 if each numbers starts with 67 and no digit appears more than once?

Explanation

There are 10 digits from 0 to 9. 67 I II III
No. of ways of filling first digit after 67 = 8
No. of ways of filling second digit after = 67 = 7
No. of ways of filling thrid digit after 67 = 6
There total number of telephone numbers
`= 8 xx 7 xx 6 = 336`