First you find 80% of 90. Set up the proportion:
80 X
—— = —-
100 90
I’m simplifying the 80/100 so it’ll be easier to solve:
4 X
— = —-
5 90
Then cross multiply:
4(90) = 5x
360 = 5x
x = 72
So 80% of 90 is 72. (Or you could have multiplied 90 by 0.8 as a shortcut). Now you find 15% of 72. You can either multiply 72 by 0.15 or set up the proportion like this:
15 X
—— = —-
100 72
Cross multiply and solve. You should get 10.8
