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....
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".
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.
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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.



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