Up to 10% of your marks can be easily lost due to carelessness. Because of exam nerves we are always careless in exams so we need to learn to check properly.
The first place to learn to check is when you do your homework and classwork. Get in the habit of checking - too many students get lazy and just ask the teacher to tell them their mistake. Once you have learnt to check in lessons, it is easy to check in exams.
In the end if you cannot find the mistake you must ask the teacher but look for it yourself first.
Check each line in a maths solution looking for any mistakes. When you have written it down, check. Always do calculations twice.
When you have finished each part of the question, go back and check.
When you have finished your solution go back to the beginning of the question and check.
When you have finished the questions you had noted (not the bonus questions) - remember this page?. Go back and check through all your questions. These are the ones that are going to give you nearly all your marks.
Do your bonus questions.
If you can gain 10% by good checking, then in an exam of 2 hours you have more than 10 minutes to check - see Time is Marks