how do i solve number series questions ?

Number series questions comes under the topic of progressions and series in CAT. First you have to find the pattern with which the question is made. Then following the same pattern we can find the remaining number/expression in the series. It comes only with practice so practice lot of questions for CAT on progressions and series. There is an concepts and formulae sheet on Progressions and series, downloading it and learning will help you to solve these questions in a better way.