Puzzles - Missing letters puzzles

Starting in the top left of the diagram, and working in columns, from top to bottom, letters follow the alphabetical sequence, advancing 2 letters, then 4, then 6, then 8 etc.

Start at the top of the diagram, and move clockwise. Letters appear in alphabetical order, in steps of 10 letters at a time.

In each diagram, add the numerical value of the upper right and left letters to the lower right and left letters, to give the letters in the centre of the columns. Calculate the difference between these middle letters to give the value of the letter right at the centre.