This is purely a question of inducing the problem solver, and the question itself is illogical. How to give an appropriate answer? Let's look at the first sentence of the question. "The sum of three people's negative 90 yuan is 270 yuan", that is, the total amount of money that three people really took out at last. But after the problem came out, 20 yuan tried to make up for 300 yuan, confusing different situations. After the waiters returned their respective 10300 yuan, this initial total no longer exists.

The process should be like this: at first, three people took out 300 yuan and gave it to the hotel, and each person spent 100 yuan (r:- 100 yuan). For the hotel, they got 300 yuan (b:+300 yuan), and then the hotel returned it to 50 yuan (b:-50), and the waiter took f:+20 yuan (.

For each customer (r:-100+10 =-90);

For hotels (b:+300-50 =+250);

For the waiter (f:+20);

Customers are "-",hotels and waiters are "+",and finally: -90×3+250+20=0.