'You did well' vs 'You did good'

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Mar 17th 2012 06:24
Today we had a final exam (It was horribly difficult, by the way). After finishing the exam, our classmates sent me text messages to say 'Have a good break'. When I tried to reply back to them 'Thank you, and how was the exam? I believe you did well'. But I confused that should I say "you did good"?
I remember I heard my friend (American) told me that "you did good".
I think "You did a good job" or "You did well" is grammatically right. Is there anyone can explain this?
Thank you so much and have a nice day!