1. We assume, that the number 20 is 100% - because it's the output value of the task.
2. We assume, that x is the value we are looking for.
3. If 20 is 100%, so we can write it down as 20=100%.
4. We know, that x is 90% of the output value, so we can write it down as x=90%.
5. Now we have two simple equations:
1) 20=100%
2) x=90%
where left sides of both of them have the same units, and both right sides have the same units, so we can do something like that:
20/x=100%/90%
6. Now we just have to solve the simple equation, and we will get the solution we are looking for.
7. Solution for what is 90% of 20
20/x=100/90
(20/x)*x=(100/90)*x - we multiply both sides of the equation by x
20=1.11111111111*x - we divide both sides of the equation by (1.11111111111) to get x
20/1.11111111111=x
18=x
x=18
now we have:
90% of 20=18