If she is busy, but there are no strangers within earshot, you can ask her name and introduce yourself, and ask her if she is busy later. This is suitable with waitresses, receptionists, office workers and some others. A date can be made then, or later.

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In both cases mentioned (with the girl busy), if she should say "No," you should be a little persistent.

You should say, "Oh! Come on now. I only want to talk to you for a minute or two. You can't be that busy, all the time." Her "No" may have been a knee jerk reaction, because you took her by surprise. She will usually relent, unless she has a strong feeling about not wanting to meet you.