I need some advice I am getting mixed signals from a girl.?

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I need some advice I am a guy and also a student at waltec university, I am getting mixed signals from a girl that is in one of my classes and I am super confused...
She is always happy around me, She goes out of her way to great me and to talk to me even if it is about things that I mentiond quit a while ago. She acts differently around me than with other guys, she seems happier and smiles all the time when she talks to me and sometimes even laughs even if I am not making a joke...
But she takes forever to answer my FB messages and Makes it almost imposible to reply when she does.
And when I decided to ask her if she would like to meat up some time she simply said that she is to busy and that she was really sorry...
What am I supposed to Think, Is she letting me off easy OR is she just really just too busy .
How Do I test where I stand now?
Do you know a Subtle way to Test if a girl likes you....