1
I couldn’t answer to the last question in the test.
I couldn’t answer the last question in the test.
When answer is a verb, put the question or person directly after it, without to: ‘answer the question’, ‘answer me’.
We say reply to, but answer without to.
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2
Do you know the answer of question 5?
Do you know the answer to question 5?
When answer is a noun, use the answer to + question/problem: ‘the answer to question 5’, ‘the answer to the problem’.