I like that you left How you did it on there. When I was in school I did it the same way. I would assume you’re using is over of equals percent over 100. The problem however is that the above number is always the total which means it should be on the bottom.
X/72= 45/100. Similar to what you have written. Solve the proportion. 100x=72x45
100x= 3240
X = 32.4