You need a little basic math knowledge to understand what's happened here. The diner has worked out how much the tip to their server should be, and come away with a figure of roughly fifteen percent. That's a little low, but it's reasonable.
If they'd tipped higher, though, they wouldn't have been able to use the Greek letter Pi to symbolize the value of their tip. The math checks out - the amount of tip required to get the bill to $30 is the value of Pi - but it's a little bit pretentious.