How many 6-digit telephone numbers can be constructed with digits 0,1,2,3,4,5,6,7,8,9 if each number starts with 35 and no digit appears more than once?

Explanation

Out of the given 10 digits, the two digits namely 3 and 5 are reserved for the two extreme left places of the desired numbers.
The remaining 8 digits can be arranged in the remaining four positions in ` ""^(8)P_(4)=(8xx7xx6xx5)=1680` ways.
Hence, 1680 telephone numbers can be constructed.