If you want to know what 90 plus 20% is, then: 20%=.2, so you multiply 90 by 1.2 = 108

If you just want to know what 20% of 90 is, then you multiply 90 by .2 = 18

If you just want to know what 20% of 90 is, then you multiply 90 by .2 = 18