Find the sum of all possible product taken 2 digit at a time by using 2, 3, 4, 5 will be?

Question

How many 5 digit numbers can be formed by using 3, 4, 5, 6, 7, if 6 is necessarily taken at hundred's place and 4 is not allowed at unit's place?
Note : Repetition of numbers is not allowed.

Answer 1

2*3
2*4
2*5
3*4
3*5
4*5
= 71