How to solve 20+ questions in quant in 20 minutes?

I hope you are talking about banking exams because in CAT it is not advisable to be in such a hurry.
Banking questions are mostly simple application of formulae. Sometimes people tend to get into equations and waste a lot of time. If you want to solve 20 questions in 20 minutes, you have to develop that art of using the options. It will only come with regular practice. Once you get used to using the options, you can manage 20 questions in 20 minutes but with conventional approaches, it is slightly difficult.