16 is what percent of 64

To get the solution, we are looking for, we need to point out what we know. 

1. We assume, that the number 64 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 64, so we can write it down as 100%=64. 
4. We know, that x% equals 16 of the output value, so we can write it down as x%=16. 
5. Now we have two simple equations:
1) 100%=64
2) x%=16
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%=64/16
6. Now we just have to solve the simple equation, and we will get the solution we are looking for.

7. Solution for 16 is what percent of 64

100%/x%=64/16
(100/x)*x=(64/16)*x       - we multiply both sides of the equation by x
100=4*x       - we divide both sides of the equation by (4) to get x
100/4=x 
25=x 
x=25

now we have: 
16 is 25% of 64