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.