Total * * * time 16 seconds. The specific idea is as follows: knock three times, the elapsed time interval is: 3- 1=2, * * * takes 4 seconds, then knock once: 4÷2=2 (seconds); Knock 9 times at 9 o'clock, the elapsed time interval is: 9- 1=8, * * takes 2×8= 16 seconds, according to which the answer is 4÷(3- 1)×(9- 1), and the final calculation is made.

As a simple math problem, the test center of this problem focuses on the rational use of division and the knowledge point that the number of time intervals = the number of knocks-1 Only by mastering it reasonably can we answer it quickly.

Matters needing attention in learning division of labor:

1, understand the meaning of division and learn how to read and write the division formula.

Multiplication is related to division, so you must memorize multiplication first.

3. Do more exercises to cultivate the sensitivity to the formula.

4. When practicing, try to use simple methods to calculate first, such as vertical calculation, algebraic division, try to simplify, decompose factors, and subtract points before calculating.