Complete the series choosing the missing number:

324, 108, 36, 12, ______

Which is the number that comes next in the following sequence?
4, 6, 12, 14, 28, 30, ?

A series is given with one term missing. Choose the correct alternative from the given ones that will complete the series.

JI, LK, NM, ? 

A series is given with one term missing. Select the correct alternative from the given ones that will complete the series.
AY, CW, EU, ?

Complete the following number series.
10 , 17, 26, 37, ____

Find the missing number in the given series.
1, 3, 9, 31, ___ 651

Complete the series choosing the missing number:

3, 15, 63, 255, 1023, ______

Which of the following letter-clusters will replace the question mark (?) in the given series?
 PK, GT, XC, OL, ?