It helps to know that 10% of anything is the number divided by 10. So 10% of 90 = 9. As it is 20% and not 10, just do it twice and add them together :) OR just times it by 2 which is much easier. So 9 x 2 = 18.
10% of a number is the number divided by 10.
5% of a number is the number divided by 10 THEN divided by 2.
1% of a number is the number divided by 100.
