ISI Kolkata does not provide a B.Math program. The majority of B.Stat students return for M.Stat while some people join CMI, TIFR, IIM, and other such organizations. 

M.Stat employees earn an average of 16–17 LPA, and many of them attend prestigious US graduate institutions. There are no fees for the entire program, and a stipend is provided. So, if you're looking for a career here, your future is bright.

Taking a course at IIT is just as difficult and demanding as pursuing a B.Stat course at ISI. They are whatever you choose to do with your life, whether you like mathematics/statistics enough to spend three years learning statistics (and a little math) and statistics solely at ISI, or you believe you are skilled in another subject that has a good course at one of the numerous IITs.

It depends totally on where you want to be. Furthermore, even if you prepare for college X, there is no guarantee that you will be accepted. As a result, keeping colleges Y and Z in mind as a backup is always a good idea.

Only you can decide which option is preferable for you and what you want. 

My brother appeared for the interview last year, so I think I can answer your question.

The panelists will evaluate your ability to explain new questions. They might even ask you to solve the questions on the board. You must be clear about your choice of ISI Kolkata as you will be asked the reason for the same during the interview.

Expect questions on topics such as Calculus, Geometry, Combinatorics, and Probability. Keep your basics clear, know the definitions, and practice solving problems based on the definitions.

Just keep practicing, be confident and give your best shot at the interview.

There are three sections in the entrance exam and the level of maths is the same as what you studied in your 12th. The exam follows the pattern of most high school mathematics competitions (RMO, INMO, etc).

The three sections of the Entrance Exam consist of MCQ, A Subjective Test, and an Interview.

The books you must know are:

1. Problem Solving Strategies by Engel Arthur
2. Test of Mathematics at the 10 + 2 level, Indian Statistical Institute, Published by Affiliated East-West Press Pvt. Ltd.
3. Challenge and Thrill of Pre-College Mathematics by B. J. Venkatachala

You should complete these all and do previous year’s papers of ISI/CMI RMO, INMO and IMO.

For Interview:

• Once you crack the exam, the interview part won’t be that difficult.
• Answer the questions confidently and honestly.
• The entrance exam of ISI is not easy and also there are only a few seats for B.Stat. 
• You have to prepare well.

Do not overthink and do your preparation well. This will help you a lot. Be calm and try to think of cracking the exam and the latter part would be an easy task for you.

The path you want to get into is a very dangerous one and it can totally spoil your career without you being able to do anything for it. 

ISI, B.Stat is a very famous course and is known for the entrance paper which is considered very tough by all those appearing in it. The course is tough because the student who is above the cutoff receives a huge financial help from the government covering the major portion of the tuition fee. Apart from this after the entrance exam the candidate can be certain of a well paid job one he/she completes the course and is awarded with the degree.

It gets difficult for the selections if there’s a year gap as lower age students are preferred and in your case there's a two year gap so the chances reduce even more. It would be very risky to change your college again and could cost some harm in your career.

Kolkata is one of India's safest cities. This city attracts visitors from all over India and the world. That's why it's known as the "City of Joy." Bengalis are extremely friendly. 

ISI Kolkata is known to have the most sought-after institute in Statistical expertise. It has one of the best faculty members in India. Out of which, 30 professors were published in Standford University.

Moreover, you have got a seat in B.Stat which is considered one of the toughest examinations for Mathematics in Asia. You should base your judgement on the basis of a stereotype as letting go of this opportunity will be a wrong decision.

Almost 50% of the batch consists of Non-Bengalis here at ISI Kolkata. There is no need to worry as the institute also has a rigid anti-ragging policy.