Y, B, T, G, O,?

The number of letters in the terms of the given series increases by one at each step.

The first letter of each term is two steps ahead of the last letter of the preceding term.

However, each term consists of consecutive letters in order.

The series may be divided into groups as shown:

b e d / f ? h / j ? l

Clearly in the first group, the second and third letters are respectively three and two steps ahead of the first letter.

A similar pattern would follow in the second and third groups.

The given sequence is a combination of two series:

I. Z, W, T, Q, ? and II. S, O, k, G, ?

The given sequence is a combination of two series:

I. C, F, I, L, O, ? and II. Z, X, V, T, ?

The given sequence is a combination of two series :

I. 1st, 3rd, 5th, 7th, 9th, 11th terms i.e. A, B, C, D, E, ? II. 2nd, 4th, 6th, 8th, 10th terms i.e. B, D, F, H, ?

Clearly, I consists of consecutive letters while II consists of alternate letters. So, the missing letter in I is F, while that in II is J. So, the missing terms i.e. 10th and 11th terms are J and F respectively.

Note that the numbers representing the difference between the consecutive terms of the series again from a series - 2, 5, 10, 17, 26, 37, 50 - in which the pattern is +3, +5, +7, +9, +11, +13.