Reasoning type of questions deals with our ability to get conclusions from the given statements of hints. The given statement may not be true by sense. But we need to believe that the given statement is true and by believing that truth we should take conclusions. It actually test our logical ability. Gate coaching center Kochi try to develop the logical reasoning ability by doing various types of problems.
The reasoning type of questions can be classified into verbal reasoning and nonverbal reasoning. Series completion type questions are more common in verbal reasoning type. In series completion questions some sequence may be given and three or four components of that particular series will be shown. We need to complete or find the next component or missing component. 
From the components given we need to crack the logic behind the series. For eg 2, 4, 6, this series is given. We need to know how 4 is obtained from 2 and how 6 is obtained from 4 likewise. We need to identify the logic behind the series. Some series may have connection in alternate terms. For e.g. 4, 3, 8, 6, 12, 9… in this series alternate terms are connected.
There is no set rule or set pattern for this type of question. The logic and sequence we need to identify using our common sense and reasoning ability. The series completion may include alphabet series, letter series, number series etc. the alphabet series may be in regular order like skipping each number of alphabets from between alphabets. For e.g. A, C, F, H… The skipping number can get increased in a series. For e.g. skipping one alphabet first then two alphabet and then three likewise. For e.g. A, C, F, J… It can also be in decreasing order.
The number series can consist of different patterns. Series of squares, cubes, square roots, cube roots, and etc. series may consist of numbers with a common difference between them. Arithmetic progression, geometric progression, are series with common difference and this common difference may be a number in case of AP and may be a ratio in case of GP.
Mixed type of series can also be there in questions. Some series may have numbers where the digits are connected. For e.g. 475., 586, 695,. Here in this series each digits of each number is connected likewise many pattern can arise in this series completion type questions. All we need to do is identifying the logic behind it.