The number 45% is what percent of 180

1. We assume, that the number 180 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 100% equals 180, so we can write it down as 100%=180. 
4. We know, that x% equals 45 of the output value, so we can write it down as x%=45. 
5. Now we have two simple equations:
1) 100%=180
2) x%=45
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:
100%/x%=180/45
6. Now we just have to solve the simple equation, and we will get the solution we are looking for.

7. Solution for 45 is what percent of 180

100%/x%=180/45
(100/x)*x=(180/45)*x       - we multiply both sides of the equation by x
100=4*x       - we divide both sides of the equation by (4) to get x
100/4=x 
25=x 
x=25
45 is 25% of 180