So you have to decrease £30 by 20%
To do this, an easy way would be to first take away 100% from 20% because you are DECREASING :
100 - 20 = 80
Then divide your answer by 100 :
80 / 100 = 0.8
This means 0.8 is going to be your multiplier so you do
0.8 * 30 = 24
Answer = £24
