The number of seven-digit numbers, with sum of the digits equal to 10 and formed by using the digits 1, 2 and 3 only, is:

Options

A. 55
B. 66
C. 77
D. 88

Explanation

A seven digit number using 1, 2, and 3 and having 10 as the of digits can be formed either by using digits 1, 1, 1,1,1,2,3or,1,1,1,1,2,2,2.
So, required number of seven digit numbers
`=(7!)/(5!)+(7!)/(4!3!)=42+35=77`