What letters will fill the blank?

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

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

13, 19, 32, 51, 83, ?

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

57, 54, 48, 39, ?, 12

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

23, 23, 46, 138, 552, ?

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

23, 19, 15, 11, ?

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

14, 14, 21, 42, 105, ?

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

42, 47, 52, 57, ?, 67

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

43, 58, 88, 133, ?, 268

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

67, 80, 93, ?, 119, 132