1 = 1*2 i.e. 2 4 = 4*5 i.e. 20 6 = 6*7 i.e. 42 9 = 9*10 i.e. 90 10 = 10*11 i.e. 110

answer is 110 n*(n+1) 10*11=110

If:

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

Then

9= ??