Your reasoning makes sense, the preposition is already in the question, so it sounds odd to include it in the answer. I'm guessing that the purpose of this exercise is to identify whether you need a preposition, so in 9-13 they probably want you to add it. Unfortunately, to my ears, the answers to 9-13 sound good both ways, though the preposition makes the answers sound a bit more formal.
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Vorpar
I'm guessing that the purpose of this exercise is to identify whether you need a preposition, so in 9-13 they probably want you to add it.
I prepared this exercise myself. I wonder why we do not need the preposition in 9-13. Can we generalize this as "We do not need the preposition in the answer, whether it be in the question or not."?
A: Whe
Linguaphile
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9.
A: When's your birthday? In American English, the affirmative sentence would also be common without the word 'on': My birthday i