Algebra and number theory provide the major chunk of questions in CAT 2013 QA section – 55-60%.
Topics that you need to look at QA are Permutations and Combinations, Probability (very basic, including die and card problems and perhaps Bayes’ theorem), Functions, Progressions (A.P, G.P. H.P. and A.G.P), Logarithms, Equations (Quadratic and Linear/Simultaneous) and most importantly, Number Theory.
Number Theory problems are usually very simple. They require certain tricks that you can pick up from any good textbook for QA
Number theory contributes 3-4 questions to every CAT, and so it is a very important topic. You should be comfortable writing numbers in their algebraic form (e.g. a three digit number having digits xyz can be represented as 100x + 10y + z). You should also learn about divisibility tests for QA.