Had dinner with a couple of friends yesterday. One of them had a $25 Groupon which he had paid $12 for. When it came it figuring out who owed what and how much, the Groupon math uncertainty was in full force. I paid what I thought was right and went home.
On the drive back I questioned my math and wondered if I somehow messed up. Turns out I had not, but my method to my total was suspect so I decided to deal with this once and for all.
It's pretty damn easy. Just subtract your share of the Groupon (in my case, this was 1/3) from your share of the total + tax + tip and that's how much you owe your friend. For example:
$22.50 my total + tax + tip
- $8 Groupon (1/3 of $25)
I owe $14.50