How to solve maths

So I'm in garde 10 Icse and have a maths exam coming up that consists of 13 chapters. I have Both Selina and Ml Aggarwal. How do I solve everything in a such a short amount of time. If not everything is there a strategy for solving minimum questions for maximum marks. Like I take 4 hours just to solve a simple chapter like shares and dividends ( because I solve everything) I just want to know a strategy where I can solve a chapter in less time but still solve all the imp questions