15 minutes? Bro that's a long time to invest and not even know how you did. Were you wanting to number close? 5 minutes should be more than enough...
Girls who are studying give signals when they want to be approached. You gave virtually no detail, so it's hard to help out, but I'll tell you what I've noticed.
If a girl is working out or studying or doing something where she should be concentrating, but you notice her eyes wandering or she looks up every few minutes, etc, then she's a prime target for an approach!
Openers are important though. Describe the approach. What'd you say, how'd the conversation go, etc. It's hard to judge her IL from the little information given.
DJZ: That would've been a good way to get her name. I usually like letting girls ASK for my name or tell me theirs; it's a good indicator of interest.
CrazyKid: From experience, if you walk up to a girl and say Hi I'm YourName, she'll usually realize it's a pickup. You made it to obvious. It might work for some, but I've found the BEST approaches are casual, spontaneous ones. Where you comment on what's going on or what she's doing. That way she doesn't smell pickup and her bit3h shields don't even come up.
A lot of times if you just walk up to a girl and don't make it seem casual, spontaneous, and spur of the moment, but rather it seems as if you're just trying to get her number, the girl will put up the shields and be like wtf is this guy talking to me?!?!
Just my 2 cents.
Cgraz
