We assume, that the number 90 is 100% - because it's the output value of the task.
We assume, that x is the value we are looking for.
If 90 is 100%, so we can write it down as 90=100%.
We know, that x is 25% of the output value, so we can write it down as x=25%.
Now we have two simple equations:
1) 90=100%
2) x=25%
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%/25%
