20 cents. The difference was just 20 cents.
If the bill was $18, as you said, it's easy to do 15% in a couple of seconds in my head . . . 10% is $1.80 and 5% is half that, or $.90 = $2.70.
You said you left $2.50.
Ergo . . .
Still, I'm guessing you left yet another piece of info out of this, that the 15% tip was mandatory and thus itemized on your bill, and thus the total bill was printed out and staring you square in the face, and you underpaid.
Else, when the guy came after you, you would have just, rightly, laughed in his face.
Amirite?
"I had $20 and a quarter on me"