1. She wrote with a pencil.

Question

2. She wrote in pencil.

3. She is writing a letter in pen.
4. She is writing a letter with a pen.

5. She is writing a letter in a pen.
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Are they all grammatical except #5? Why can't we put 'a' or 'an' after 'in' in each sentence?

Answer 1

1 - 4 are fine.
5 is incorrect. Use 3 instead.