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If you are talking to someone you are framiliar with, family member, school mate or such, we don't say "вы правы." If it were a girl- Ты права. If it were a boy- Ты прав. Though if it were someone...

When you are to use ты it can be for close friends, family, children that are younger than you, etc. People you are framiliar with, or that are younger than you.

For вы it is for formal adress....