To form a seven digit number with sum of digits 10 , all the digits can't be 1, 2 or 3 . Hence, seven digit number must have the following cases,
Case 1. Using 1, 1, 1, 1, 1, 2, 3
Possible seven digit numbers will be
==7×6=42Case 2. Using
2,2,2,1,1,1,1Possible numbers will be
===35 No more cases will be forme(d)
Hence, total number of seven digit numbers possible
=42+35=77