If you are preparing for IIT, practicing JEE Advanced Mock Test (https://www.selfstudys.com/mcq/jee/online/mock-test/jee-advanced) and Previous Year Question Papers (https://www.selfstudys.com/books/jee-previous-year-paper/english/iit-jee-advanced-previous-year-paper) is a must. Mock tests help improve time management and accuracy, while PYQs give a clear idea of exam pattern and important topics. I personally saw a huge improvement after solving them regularly.