Do I need the prepositions in these sentences?
The exam sheet should only be used to write answers (on).
The blanks should only be used to write answers (in).
The equipment should only be used to test the operation of the system (with).
I feel either way works but it seems to me it is more common to omit the preposition. The versions with prepositions sound better to me though. Are there situations where the prepositions can't be omitted? What rules govern this?