Ok, we have

11: 1x1=1, 11 is a multiple of 1

12: 1x2=2, 12 is a multiple of 2

24: 2x4=8, 24 is a multiple of 8

36: 3x6=18, 36 is a multiple of 18

Does this make sense?

If so you can now find your 5th number, there's not that many to choose from i.e. between 10 & 99