WFB, TGD, QHG,?

So, 521 is wrong and must be replaced by (498 + 3³) i.e. 525.

So, 5686 is wrong and must be replaced by (4566 + 1110) i.e. 5676.

So, 202 is wrong and must be replaced by (259 - 55) i.e. 204.

I. 0, 3, 8, 15, 24, 35 and II. 2, 5, 10, 18, 26

The pattern in both I and II is + 3, + 5, + 7, + 9,.....

So, in II, 18 is wrong and must be replaced by (10 + 7) i.e. 17.

So, 86 is wrong and must be replaced by (78 + 9) i.e. 87.

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

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.