Five customers buy 10, 15, 20, 25, 30 toffees respectively from one shop. But the person who bought 20 toffees forgot to pay. If the shopkeeper makes 20% profit, what will be his profit or loss from these five customers if the person who forgot to pay had paid his amount?.

It depends on the subject for me. some exams are easier than others so i usually go through it and answer questions i know based on my general knowledge, then double check after. going through the lesson can a lot  because the exams can be  very lesson-specific, but sometimes they are ridiculously long so a lot of people don't take the time. i did find that, when i went through the lessons, i was making 100's on my exams vs strictly using + the internet and  i make 90's.