Puzzles - Missing letters puzzles

Answer

:

O

Explanation

:

In each row, letters follow the alphabetic sequence, in steps of 4 letters each time for the first row, 5 letters for the second, then 6, 7 etc.

Answer

:

H

Explanation

:

In each row of the diagram, the numerical values of the letters add up to 26.

Answer

:

X

Explanation

:

In each circle, start with the top left segment, and move clockwise around the other segments. The value of each letter increases by 5 for the first circle, 6 for the next, then 7, 8 and 9.