Norwegian 5 - Lesson 6

I find the following in the above mentioned lesson:

“de er flinke til mange ting” meaning “they’re good at many things” and
“de er for eksempel skikkelig flinke i matematikk” meaning “they’re for example pretty good at maths.”

Why is it that they use the word “til” for “at” in the former sentence and “i” in the latter?

Why is it wrong to say “de er flinke til matematikk?”

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This is about prepositons, the saying is ‘være flink i noe’, and also ‘være flink til noe’. They mean different things. The first one is about being good at something, the other is about being good at doing something. So ‘de er flinke til matematikk’ is wrong since it’s not about doing something (in Norwegian). We don’t ‘do math’.

As per this comment, we’ve changed the English definition slightly to clarify the differences as well.