Other people are getting the reverse rule wrong. It would not be twice your age minus seven; it would be twice (your age minus seven).
- Dude, you're 40 and dating a 27-year-old. Isn't that a bit gross?
- Hey, half-your-age-plus-seven.
- Oh? So that means you would go for twice-your-age-minus-seven? So for you that would be 73.
- Actually, it's twice-the-quantity-your-age-minus-seven. So 66.
- I'm guessing you're too dense to see this algebraically. It's alright; we can take our results and work backwards. 73/2=36.5, and 36.5+7=43.5. However, 66/2=33, and 33+7=40. Got it?
- Ah, OK. So you'd go for a 66-year-old then?
- Fuck no.