Many universities have admissions tests and interviews that involve solving problems.
In the area of physics and mathematics the Isaac Physics website provides an opportunity to practise the skills needed for such problems, and will be particularly helpful for students who are interested in studying Natural Sciences (Physical) or Chemical Engineering via Natural Sciences. You may find the Core Maths for All Scientists section particularly useful.*
Of course , if you're invited for interviews at Cambridge, do remember that you may be asked questions on a wider range of science than is presented on the Isaac Physics website. The interview film on this page provides a good introduction to Cambridge interviews.
*in A level terms, this section focuses on the material from modules C1, C2 and M1.