: Remember to tip 18%, everybody.

** Mathematician**: Is that 18% of the

*pre*-tax total, or of the total

*with*tax?

** Physicist**: You know, it’s simpler if we assume the system doesn’t have tax.

** Computer Scientist**: But it

*does*have tax.

** Physicist**: Sure, but the numbers work out more cleanly if we don’t pay tax and tip. It’s a pretty small error term. Let’s not complicate things unnecessarily.

** Engineer**: What you call a “small error,” I call a “collapsed bridge.”

** Economist**: Forget it. Taxes are inefficient, anyway. They create deadweight loss.

** Mathematician**: There you go again…

** Economist**: I mean it! If there were no taxes, I would have ordered a second soda. But instead, the government intervened, and by increasing transaction costs, prevented an exchange that would have benefited both me and the restaurant.

** Engineer**: You

*did*order a second soda.

** Economist**: In practice, yes. But my argument still holds in theory.

*The computer scientist lays a smart phone on the table.*

** Computer Scientist**: Okay, I’ve coded a program to help us compute the check.

** Mathematician**: Hmmph. Any idiot could do that. It’s a trivial problem.

** Computer Scientist**: Do you even know how to code?

** Mathematician**: Why bother? Learning to code is also a trivial problem.

** Engineer**: Uh… your program says we each owe $8400.

** Computer Scientist**: Well, I haven’t de-bugged it yet, if that’s what you’re getting at.

: This is a waste of time. Let’s just split it evenly.

** Economist**: No! That’s so inefficient. Let’s each write down the amount we’re willing to put in, then auction off the remainder at some point on the contract curve.

** Physicist**: Huh?

** Mathematician**: Like most economics, that’s just gibberish with the word “auction” in it.

** Engineer**: Look, it’s simple. Total your items, add 8% tax, and 18% tip.

** Mathematician**: Sure. Does anybody know 12 plus 7?

** Computer Scientist**: You don’t?

** Mathematician**: What do I look like, a human calculator? Numbers are for children, half-wits, and bored cats.

*The engineer looks at the cash they’ve gathered.*

** Engineer**: Is everyone’s money in? It seems we’re a little short…

** Physicist**: How short?

** Engineer**: Well, the total was $104, not including tip… and so far we’ve got $31.07 and an old lottery ticket.

** Physicist**: Close enough, right? It’s a small error term.

** Mathematician**: Which of you idiots wasted your money on a

*lottery*ticket?

** Economist**: I should mention that I’m not planning to eat here again. Are any of you?

** Computer Scientist**: What does that matter?

** Economist**: Well, in a non-iterated prisoner’s dilemma, the dominant strategy is to defect.

** Engineer**: Meaning?

** Economist**: We should be tipping 0%, since we’ll never see that waiter again.

** Computer Scientist**: That’s awful.

** Physicist**: Will the waiter really care – 0%, 20%? Let’s not split hairs. It’s a small error term.

*The engineer looks up from a graphing calculator.*

** Engineer**: All right. I’ve computed the precise amount each of us should pay, using double integrals and partial derivatives. I triple-checked my work.

** Mathematician**: Didn’t we all order the same thing? You could have just divided the total by five.

** Engineer**: I could? I mean… of course I could! Shut up! You think you’re so clever!

: So, we’re all agreed on a 0% tip?

** Computer Scientist**: Well… the waiter did only bring two orders of fries for the table.

** Physicist**: We only ordered two.

** Computer Scientist**: Exactly. We got the 1

^{st}order, and the 2

^{nd}, but never the 0

^{th}.

** Economist**: I’ll be frank. At this point, my self-interest lies in not paying. And the economy prospers when we each pursue our individual self-interest. See you later!

*The economist dashes off. The engineer and computer scientist glance at one another, then follow.*

** Mathematician**: Looks like it’s just me and you, now.

** Physicist**: Good. The two-body problem will be easier to solve.

** Mathematician**: How?

** Physicist**: By reducing it to a one-body problem.

*The physicist scampers away.*

** Mathematician**: Wait! Come back here!

** Waiter**: I notice your friends have gone. Are you done with paying the check?

** Mathematician**: Well, I’ve got a proof that we

*can*pay. But I warn you: it’s not constructive.

*Thanks for reading! (And for your benign tolerance of my whiteboard doodles.) You might also enjoy “A Math Professor Consults on a Hollywood Movie.”*