Lizzy: Mark wants to tip his server 20% on a $200 check. If his friend agrees to kick in $10, how much should Mark add?.
Me: Hmmm, let me think. I think this is the detailed solution:
First find the total tip amount: 20% * $200 = $40. Then subtract the friend's contribution: $40 - $10 = $30. Final answer: 30.