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?”