Most of the CAT students are afraid of Quants section and the level of difficulty of this section is increasing as well. Starting from the basics, you can choose NCERT maths books going through chapters like Profit and loss, Ratio, Probability etc.

You can also attempt previous year papers to judge your knowledge of this section. Later you can jump to standard preparation books for CAT preparation. Check the list here for each section.

This shall not only help you to solve complex questions but will also help you to understand the level of a manipulation of a sum you can see in the exam. 

Later you will have to give online/offline, sectional and full length mocks to aasess your performance, analyse and also improve your speed and accuracy.

It depends on how much practice you do and invest in analysing your mock results.

Hope this helps,

Good Luck.