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Sirs

Request your help....

When I was going through "Common Mistakes in English" (Longman), I saw the following:

Good at, not in.
Don't say: My sister's good in maths.
Say:         My sister's good at maths.

But I saw the following on one Site:

When it comes to school subjects, both “good at” and “good in” are used
Jere is good at math: he always finishes first.
Jere is good in math: he makes all A’s.

Please clarify which one is correct nowadays....
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It's possible also that there are regional and/or personal differences. For me, "My sister's good in maths" seems unacceptable if "maths" means the subject. It may be feasible if "maths" means "maths classes".
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Language changes over time.
Longman is traditional, and you can't go wrong following their guidelines.
When you take a grammar test, go traditional.
But "in" is OK in modern conversations. Perhaps a newer edition of Longman will reflect this change.
Students: We have free audio pronunciation exercises.
Comments  
Thank you very much for the help. Emotion: smile
 GPY's reply was promoted to an answer.
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Following on from my previous reply, the graphs below do seem to suggest that "good in" in this sense is predominantly US.

https://books.google.com/ngrams/graph?content=good+in+math%2Cgood+at+math&year_start=1800&year_end=2000&corpus=17&smoothing=3

https://books.google.com/ngrams/graph?content=good+in+maths%2Cgood+at+maths&year_start=1800&year_end=2000&corpus=18

(I used "maths" for the British plot and "math" for US plot, reflecting the usual forms in those countries.)