Why would she ask for my number?

I started my job a couple weeks ago and this girl started working the same time. We work different departments so we only see each other a little bit. One night we got off the same time and we ended up just talking for about 30 minutes. She wanted to show me her speakers in her car so we were sitting listening to music. She wanted to show me something on her phone and then she gave me her iPhone with the new contact open and said you should give me your number. So I put my info in her phone but she hasn't texted me or called and it's been almost a week. Why would she ask for my number then? She's 18 and I'm 21. She does have a boyfriend but In not the one who asked for a number and I regret not asking for hers so I could talk to her. Would it look desperate the next time I see her if I ask for hers?

No it would not look desperate

Maybe she wants her boyfriend to think she's cheating?

She likes you. If you like her make a move that is the first step. Maybe wait until she doesn't have a boyfriend anymore. Ask her to tea or something. Go somewhere close to a park so that you can walk together after and bond.

Best of luck.

Yes thiis girls seems to be very quick dear… Its my advice to you that don't be so close to herr because the girl who is cheating with her bfrnd(by trying to get close of u) will surely one day cheat with you to get close of another person smart more than u

Ask her who cares

By asking for his number you are taking the lead in your social interactions, if you don't feel comfortable in that role it probably isn't a good idea to ask for his number. You set a president that you are willing and able to take charge. If you later want to be chased and play hard to get he will probably take that as a signal that you are no longer interested.

On the other hand if you are ready to throw away the old gender roles and embrace the 21st century then by all means go for it. Take charge and go out and pick up whoever you want.