In these questions, you have to complete the series keeping given members of the series in consideration.

Lets start with
1 3 9 27 ?

The have given us, first four members of the series and the fifth one is required.

Its quite obvious that it is a series of 3^(n-1) where n is the position of the number in the series.

3^0 = 1 which is the first member of the series
3^1 = 3 which is the second member
3^2 = 9 which is the third member
3^3 = 27 which is the fourth member
3^4 = 81 which is the fifth and the required member

Now take
6 11 20 37 ?

6 is the first member of the series
( 6 x 2 ) - 1 = 12 - 1 = 11 which is the second member
( 11 X 2 ) - 2 = 22 - 2 = 20 which is the third member
( 20 x 2 ) - 3 = 40 - 3 = 37 which is the fourth member
( 37 x 2 ) - 4 = 74 - 4 = 70 which is the fifth and the required member

Now lets move to
4 20 35 49

4 is the first member of the series
4 + 16 = 20 which is the second member
20 + 15=35 which is the third member
35 + 14 = 49 which is the foruth member
49 + 13 = 62 which is the fifth and the required member

Now we have
6 8 10 14

6 is the first member of the series
6 + 2 = 8 which is the second member
8 + 2 = 10 which is the third member
10 + 4 = 14 which is the foruth member
14 + 4 = 18 which is the fifth and the required member

Now
212 179 146 113

212 is the first member of the series
212 - 33 = 179 which is the second member
179 - 33 = 146 which is the third member
146 - 33 = 113 which is the foruth member
113 - 33 = 80 which is the fifth and the required member

Here is the last one
18 20 24 32

18 is the first member of the series
18 + (2^1) = 18 + 2 = 20 which is the second member
20 + (2^2) = 20 + 4 = 24 which is the third member
24 + (2^3) = 24 + 8 = 32 which is the foruth member
32 + (2^4) = 32 + 16 = 48 which is the fifth and the required member

In all such series, there is a pattern. One just needs practice to identify it.