1. Xiaohua bought a pencil, two erasers and two notebooks, paid one yuan and the salesman gave him five cents. Xiaohua looked at the price of a pencil and said, Uncle, you have miscalculated. Think about it. Why did Xiaohua know that the account was miscalculated so quickly?

Judging from the parity of numbers, you know that the calculation is wrong without calculation, because a pencil costs 80 cents, which is an even number. In addition, no matter how much the eraser and exercise book cost, the two erasers must be even, so the total amount of money Xiaohua has to pay should be even. He paid 1 yuan, which is 100. The money he got back was 50 cents, which was odd. You don't need to calculate it to know that the calculation is wrong.

2.

Turning on the light at night, the tiger was naughty and pulled the switch seven times in a row. Can you tell me if the light is on? We might as well keep asking, pull it eight times? How about nine o'clock? 10? Even 100 strokes? Can you know if the light is on?