"Sorry, we can't make the 9-inch cake you ordered. Would two 5-inch ones be okay?"

My friend immediately took out his phone and opened the calculator: "The area of a 9-inch cake is about 63.6 square inches, and two 5-inch cakes only add up to 39.3, which is nearly 40% less!"
The waiter was stunned: "Let me ask the manager..."
The manager came over, smiling awkwardly: "How about three 5-inch cakes?"
My friend calculated again: "Three only come to 59, still not enough."
The manager wiped his sweat: "Then four! Four should be enough, right?"
My friend shook his head: "Four is 78.5, that's too much, you're losing out."
The manager was overwhelmed: "So what do you suggest?"
My friend calmly said: "Refund the money, I'll buy from the next door."
The manager tearfully processed the refund.
After we left, I asked him, "You're so good at math. When are you going to pay back the 500 bucks you borrowed last time?"
He suddenly quickened his pace: "Uh... how about I treat you to two 5-inch cakes?"