Around 11% of their bill. I'm not 100% sure about the answer, however, the way I got it: Subtract 22.50 from 30.00 to get the amount of change they get, This is 7.50. Then subtract 5 dollars from it because the question states all of the remaining change other than the 5 dollar bill. Which leaves you with $2.5. Then divide 2.5 by 22.5 to get the final percentage of the tip from the original bill of the dinner. In which, you get 11%.