Here are a few examples:

Example 1: How many candles are left on the table?

Title: There are 12 lighted candles on the table. First three were blown out by the wind, and then two were blown out by a gust of wind. Finally, how many candles are left on the table?

Answer: 5.

Tip: What is not blown out is burned out.

Example 2: How many lamps are left?

Topic: There are 9 lights in the classroom. Three Lamps District has gone out. How many lights are left?

Answer: 9 lights

Tip: the problem is how many lights are left, not how many lights are on, so it used to be 9 lights, and now it is still 9 lights.

Example 3: Soy sauce?

Title: Xiaoming's family has 16 Jin of soy sauce, which takes away 2 Jin every month. How many months will it take to make soy sauce?

Answer: 7 months later.

Tip: If you don't give time to think about this question and ask for an immediate answer, the average person may answer: eight months later, but in fact, you will run out of soy sauce in seven months.

Example 4: Can you make the cup face down?

Title: There are 14 cups on the desktop, with three cups facing up. Now turn four cups at a time (when turning the cups, the cups are upside down and the cups are upside down).

Q: Can you put the cup mouth down after several times? If not, can I only turn it over six times at a time? How about seven o'clock?

Answer: 4 or 6 can't do it, only 7 can.

Tip: Use+1 for the cup with mouth up and-1 for the cup with mouth down.

The initial state is 3 "+",11"-",so when the number of 14 is multiplied, the product is-1, and when1cup is turned,+1 becomes-1 or.

Flipping cups for n times is equivalent to multiplying by n "- 1 ",so every time an even number of cups are flipped, it is the result of"-1 ",and the initial state is not changed.

So turning four cups at a time and six cups at a time can't change the result that the product is "-1".

However, if you flip an odd number of cups at a time, you can change the initial state and the result is "-1".

So turn seven cups at a time and turn them odd times.

The specific operation is as follows: in the original state, 3 cups are facing up, 1 1 cups are facing down;

(1) Turn 2 cups upwards and 5 cups downwards; After turning around, 6 cups are facing up and 8 cups are facing down;

(2) Flip three cups upwards and four cups downwards; After turning over, 7 cups are upward and 7 cups are downward;

③ Turn over 7 cups with their mouths facing upwards.

After the turn, all 14 cups are facing down, thus completing the task.

Finally, I attach the cover of the interesting math problem to you!