Hello.
The answer is
1. We assume, that the number 90 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 90 is 100%, so we can write it down as 90=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) 90=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:
90/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 90
90/x=100/15
(90/x)*x=(100/15)*x - we multiply both sides of the equation by x
90=6.66666666667*x - we divide both sides of the equation by (6.66666666667) to get x
90/6.66666666667=x
13.5=x
x=13.5
now we have:
15% of 90=13.5
Have a nice day