After the waiter returns 1 yuan,

Namely 10- 1=9,

Everyone only spent 9 yuan's money,

Three people each, 9 yuan,

3×9 = 27 yuan, the money has included 2 yuan, who was secretly hidden by the waiter.

Together with the 3 yuan in their hands, it is exactly 30 yuan, in fact, there is not a penny less.

2. Because if you buy onions together, whether they are white or green, they are all 1 yuan 1 kg.

When sold separately, it became 7 Mao Cong, 3 hairs lower than the original, and 3 Mao Cong, 7 hairs lower than the original.

So the onion seller lost 3x50+7x50=50 yuan.

So selling things sometimes requires knowledge. .