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%