What do you guys do when you ask a girl for a number and she doesn't explicity reject you, but also doesn't affirm it? This means that she says something like "I don't know. Now I really gotta go now to my class/meeting/etc."

Do you just let it go? Or have some of you guys been succcesful when persisting? If it's the latter: how did/should you do it?

I think this happens when you didn't botch the interaction outright but the girl is in a hurry, a cautious person, or you didn't build quite enough trust/attraction. I am talking about a daygame context.