Hello,

It is a great aim that you want to pursue BSc. in Mathematics which is tough subject and course.  yes IITs provides BS course for 4 years duration but they are mostly integrated which means you have to do your BS and MS both at the same place which will be a 5 years course.

For taking an admission to IIT you have to go through JEE advance and have to perform best. This is the only way to get into IIT college as a student.  After that you will have different procedure for different IIT college to take an admission in BS. For admission process for BS It is same as admission process to Btech.

from all the IITs, bombay campus and Kharagpur campus is good for pursuing this course. You will get wide area and resources to get your work done to a best level.

For more details you can visit the official webpage of respective IIT campus to get detailed and broad information.

Hope it helps!