Hi,
81 is 37.5% of 216.
"1. We assume, that the number 37.5 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 37.5, so we can write it down as 100%=37.5. 
4. We know, that x% equals 81 of the output value, so we can write it down as x%=81. 
5. Now we have two simple equations: 
1) 100%=37.5 
2) x%=81 
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%=37.5/81 
6. Now we just have to solve the simple equation, and we will get the solution we are looking for. 
 
7. Solution for 81 is what percent of 37.5 
 
100%/x%=37.5/81 
(100/x)*x=(37.5/81)*x - we multiply both sides of the equation by x 
100=0.46296296296296*x - we divide both sides of the equation by (0.46296296296296) to get x 
100/0.46296296296296=x 
216=x 
x=216  "
Have a great day!