Puzzles - Missing letters puzzles

Exercise : Missing letters puzzles
4.
Answer
:
K
Explanation
:
As you move down, the numerical value of the letters follows the sequence of Prime Numbers.

5.
Answer
:
A
Explanation
:
Writing each letter as its numerical value and working in rows, add the top 2 digit number to the bottom 2 digit number to give the 3 digit result in the centre.

6.
Answer
:
G
Explanation
:
The numerical values of the letters in each row add up to 26 each time.