Tell me please if I have to use the indefinite article before #nine in the following context.
On the scale of 1 to 10, how strong was the pain of having your tooth drilled. Well it was (a) nine.
If I need to use a there, could you explain why?
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In this case, the article is optional.
The response is idiomatic both with and without the a. The choice might depend on exactly what's in the respondent's mind when s/he answers.
If the respondent imagines choosing a number from the numbers presented, the answer is likely to be a nine. Similarly a contestant in a TV quiz might choose the (number) nine as the question to answer.
On the other hand, if the respondent simply thinks about the scale of pain, responses such as I would say nine and it was about nine are likely.
It's similar to asking someone throwing dice what s/he has just thrown. The response could be either I threw a six or I threw six.
In this context, the latter answer cannot be misinterpreted as six throws rather than the number six. Both responses are perfectly clear and idiomatic.