prefer to say " Apa kabar?"

Use 'kamu' for people you already know - better yet, wait until they use kamu (or kau) and follow the example. Litterally, it means "what news (with) you?" or "what (is) your news" (more correctly said as, "Apa kabar mu?")

apa kabar kamu - this is a non-formal way to say how are you apa kabar anda - this is a formal way to say how are you so it's ok to use both :)

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apa kabar anda is used in formal way... to simplify,,, just say 'apa kabar?'