The best way of approaching the number series questions to first observe the difference between terms.If the difference is constant , it is a constant difference series.If the the difference is increasing or decreasing by a constant number, then it is a series with a constant increasing or decreasing difference .If there is no constant increasing or decreasing difference , then try out the product series approach.

For this, first divide the second term with the first term and third with the second and so on.If the numbers obtained are the same, then it is a product series.Alternatively, try writing each term of the series as a product of two factors and see if there is any pattern that can be observed .If still there is no inference , but the difference is increasing or decreasing in a rapid manner , then check out the square series.If the increase is high and it is not a square series, then try out the cube series.

If the difference is alternately decreasing and increasing, then it should most probably be a mixed series.Therefore test out the series with alternate numbers.If still the series is not solved, try out the general series.