Hope this makes sense to you:

Transposing from F to A takes you up four semitones, so everything else has to go up four semitones too.

A to C is only 3 semitones, because there is only a semitone between B and C (no black note on the keyboard in between them), so go up another semitone to get to C#.

Likewise there is only a semitone between E and F, so D to F is only 3 semitones: go up another semitone to get to F#.

(I'd rather be explaining this with a keyboard in front of me- apologies if it doesn't come across well.)

Noreen