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
