2*3=6 3*4=12 similarly 9*10=90

The answer is 90.

And i used to solve this is a*(a+1)

1 = 2 2 = 6 3 = 12 4 = 20 5 = 30 6 = 42 7 = 56 8 = 72 than 9 = 90

If:

2=6 3=12 4=20 5=30 6=42

Then

9= ??

Ans= 60