1. We assume, that the number 40 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 100% equals 40, so we can write it down as 100%=40.
4. We know, that x% equals 21 of the output value, so we can write it down as x%=21.
5. Now we have two simple equations:
1) 100%=40
2) x%=21
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:
100%/x%=40/21
6. Now we just have to solve the simple equation, and we will get the solution we are looking for.
7. Solution for 21 is what percent of 40
100%/x%=40/21
(100/x)*x=(40/21)*x - we multiply both sides of the equation by x
100=1.90476190476*x - we divide both sides of the equation by (1.90476190476) to get x
100/1.90476190476=x
52.5=x
x=52.5
now we have:
21 is 52.5% of 40