Science

Well, with these marks, you are eligible to get enrolled in the counselling process.
For more details visit the link:
https://www.shiksha.com/college/st-xavier-s-college-kolkata-park-street-23242/admission

It's not possible for you to take admission in any of the top institutes as most of them have minimum eligibility as 60% in 12th. So, you have to go with low-tier B.Arch colleges.

Hi, Admission in B.Sc Hons is based on marks in three subjects
physics+ mathematics+ chemistry/ computer science. You will get admission in Delhi University but maybe not in the North campus if you belong to general category.

Yes, you can get in top colleges to pursue B.Sc in mathematics.

Hey,
You can apply application form online from Delhi University website for under graduation courses under graduation portal.
The portal is going to live this week.
Stay updated through the university website.
For more information regarding admission process, visit Delhi university website.

Generally to pursue higher level programs, the minimum percentage in 60 percent in some colleges and for top institutes, it may go up to 70 percentage.