In order to know when to simplify, you need to see if the number(s) are big and you also have to know about your time tables, like for example, on your test you got a 5/20, and you can tell that you can simplify by this by 5, since 5x1=5 and 5x4=20, so when you simplified it, you get 1/4, which is a simplified answer. So basically, you need to check if the numbers are big or small, if it's big, you need to simplify, if it is small, then you don't need to simplify.
