As always, karishma is right.
you need to know why and you also need to practice a lot.
Going through a lot of different areas and understanding the reason behind 'why' you used what you did is the key.
speed is important as well. If you are able to spend 4-6 minutes on questions because you know you can speed up later and have the belief that you won't mess up, it will give you tremendous confidence.
I believe the improvement only comes by spending enough time on each question, understanding concepts, lingering on problems, not going to the solution till you make enough of a headway into the problem, trying to solve problems in different ways etc. Obviously, all of this takes a lot of time, and you need to weigh whether it is worth the time.
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