What if you ask a girl to hang out and she asks why I asked her? (quick response please thanks )

She is probably trying to figure out if you like her or not.

You should say, "Because I like you and I'd like to get to know you better."

i think that you have to remember hwo she said it. was she grumpy? or was she just tired and stressed?
if I were writing an exam, I'd be busy studying. I'd probably ignore a few people for a bit but ask her afterwards what she thinks of it when she's not busy.

She asked why because she is trying to find out where you are coming from. When a girl asks why it's to find out if it's because you like her or not. That being said, if she does want to know if you like her or not it does not necessarily mean she likes you to. Just tell her you thought she would be fun to hang out with and see how she responds to that. If she responds positively to a compliment then you are in, if it's an iffy sort of response then she wants to let you down gently.

tell her your bored and got nothing else to
do so you wanna hang out with her.
(tell her you chose her cause she's she only person you can think of.)