20% of 90, and i have to show workings out

10% of 90 is 9, 10% times 2 is 20%, therefore 20% of 90 is 2*10% of 90, or 18

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.