Does 65-(45-20)equal (65-45)-20? how do you know?

No, it does not. 65-(45-20) is equal to 40, because you subtract 45-20 before because of the parentheses. When you do that, you get 25 then subtract 65-25, then get 40. Then, if you subtract 65-45 first and get 25, then subtract 25-20, you'll get 5.