muy bien?
or just mi bien!
I guess Duolingo is wrong then because isn't mi in Spanish im (or wait is it me if so then that doesn't make sense hmmmm)
it depends on what you want to say but if you wanted to say I'm good then it would be (soy)me (Bien)good but if you are trying to say very good that would be (Muy)very (Bien)good
