Puzzles - Missing letters puzzles

Exercise : Missing letters puzzles
19.
Answer
:
Y
Explanation
:
Starting in the top left circle, and following a W pattern through the others, letters in corresponding segments of the circles follow the alphabetical sequence in steps of 4 letters.

20.
Answer
:
W
Explanation
:
Working from left to right, letters move forwards 7 places, then back 2. Repeat this sequence until the end.

21.
Answer
:
K
Explanation
:
In each row, the product of the left and right hand numbers equals the sum of the numerical values of the three letters.