I assume, that the number 120 is 100% - because it's the output value of the task.
If 120 is 100%, so we can write it down as 120=100%.
We know, that x is 25% of the output value, so we can write it down as x=25%.
<em>Now we have two equations:</em>
<em>1) 120=100%
</em>
<em>2) x=25%</em>
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:
<em>120/x=100%/25%</em>
<em>Solution for what is 25% of 120:</em>
<em>120/x=100/25
</em>
<em>(120/x)*x=(100/25)*x - we multiply both sides of the equation by x
</em>
<em>120=4*x - we divide both sides of the equation by (4) to get x
</em>
<em>120/4=x </em>
<em>30=x </em>
<em>x=30</em>
<em>The answer is: 25% of 120=30</em>