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ally rule
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I think he means that your average person at a bar would be in his 20's, so if you subtract 15 years you'd just end up as a child or teenager. You'd have to be at least 33 years old for it to be a story about adults.
I mean the joke wouldn't work anyway, everyone would know the girl just gave a fake number or is pranking. The intended "ghost" effect is just inside OPs head.