Iwould like to know 15 is what % of 39

Setup
remember is means =, and of means *

Writing across the words
15 = x * 39
15/39 = x
x =0.3846 = 38.46%

2. We assume, that x is the value we are looking for.
3. If 39 is 100%, so we can write it down as 39=100%.
4. We know, that x is 15% of the output value, so we can write it down as x=15%.
5. Now we have two simple equations:
1) 39=100%
2) x=15%
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:
39/x=100%/15%
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 15% of 39

39/x=100/15
(39/x)*x=(100/15)*x - we multiply both sides of the equation by x
39=6.66666666667*x - we divide both sides of the equation by (6.66666666667) to get x
39/6.66666666667=x
5.85=x
x=5.85
now we have:
15% of 39=5.85