Why is it 'good at maths' but 'good in sports'? Which preposition is correct?

The standard collocation is 'good at' when you talk about a skill or activity: good at maths, good at chess, good at painting, good at swimming. So 'good at sports' is the safest exam choice. 'Good in' is mainly used with a place or situation, like 'good in a crisis' or 'good in the kitchen', meaning you perform well in that setting. For abilities and subjects, always prefer 'good at'. The same pattern follows for 'bad at', 'clever at' and 'brilliant at'. If you are ever unsure in the exam, 'at' is correct for skills almost every time, so write 'good at sports' and 'good at maths' confidently.