What does this idiom mean: 'to be red hot' at something?

[Paul]

Test No. incompl/inter-2 "Like a red rag", question 9

Ask her to help you with your calculations because she's ......... hot at mathematics.

(a) red
(b) blue
(c) white
(d) black

Test No. incompl/inter-2 "Like a red rag", answer 9

Ask her to help you with your calculations because she's red hot at mathematics.

Correct answer: (a) red

Your answer was: correct
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dear teacher,
Could you pls explain this sentence
Thanks

[Alan]

This means she was very good at mathematics. If you are red hot at something it means you are the best and have no difficulty at all in doing it.
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English as a Second Language
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