Learn English – the difference between “refer the letter” and “refer to the letter”

(AmE) Chocolate is good in cake. Sarcasm can be good in the proper context. That actor was good in his last movie. Wine is good in moderation. She's good in bed. What's good in this restaurant? Was there some good in these examples?

"good at" however, means adept at performing a particular activity, whether physical or academic. She's good at math, but he's good at sports. I think I'm good at English grammar. I know I'm good at solving crossword puzzles. I hope I was good at explaining to you.

According to this author there's a subtle difference between the two uses:

be happy with = be satisfied with something you possess or have experienced;

be happy about = be pleased by; be glad that something happened.

However, the author concludes:

Nevertheless, you will hear some native speakers use the two expressions quite interchangeably, so even if you confuse them, you will likely be understood.

This said, if you want to be as correct as possible, you should treat the subject with due care.