Hello Naseem, as you are saying to leave 20 marks maths portion then you are already a step backward in race because all of them are having 200 marks to score in and you will left only 180 to score in. its not that you dont  have maths so leave it if you try to do from 20 atleast 9 or 7 can be solve and which are not time consuming because you have one more reason for leaving maths is time but if you leave maths portion then you are giving your marks in free on your own and one marks can give a big difference in colleges so my suggestion is to not to leave any portion of the paper try to attempt everything because everything is equally important. start practising regularly study hard

Good Luck