1. He is writing a letter with a pen.

Question

2. He is writing a letter in pen.

3. He is writing a letter with a ball-point pen.
4. He is writing a letter in ball-point pen.

5. He is writing a letter with a fountain pen.
6. He is writing a letter in fountain pen.
[Is each pair grammatical? Why is 'a' not used after the preposition 'in'?]

Answer 1

Each pair is fine and each pair means the same thing.