What letters will fill the blank?

Z, X, V, T, R, __,__

In the following question, select the missing number from the given series.

6, 11, 16, 21, 26, ?

In the following question, select the missing number from the given series.

91, 100, 118, 145, ?, 226

In the following question, select the missing number from the given series.

29, 27, 24, ?, 15, 9

In the following question, select the missing number from the given series.

6578, 4381, 2653, ?, 322

Choose the correct alternative which will complete the following series.

4, 8, 12, 24, 18, ?

Find the missing (?) in the series.

3, 5, 9, 11, ?, 33, 65,...

In the following question, select the missing number from the given series.

87, 62, 42, 27, 17, ?

In the following question, select the missing number from the given series.

11, 13, 17, 19, 23, ?