12, senior one * * * held 24 math exams, and * * * gave 426 questions, 25 questions each time, 20 questions, 16 questions. Q: How many times has the test with 25 questions been held? Test site: the application of ternary linear equations. Special topic: classified discussion. Analysis: First, suppose that the number of questions in 25, 20 and 16 are x, y and z respectively.

According to the meaning of the question, list the equations and find x= by adding, subtracting and eliminating. According to this formula, observe the values of z, x and y of symbolic conditions, and then x is the demand. Solution: Set the times of 25 questions, 20 questions and 16 questions as x, y and z respectively.

Judging from the meaning of the question.

4z-5x=54 from ①×20-②, that is, x= ③.

It can be seen from ③ that the unit number of 4z is 4 or 9, and 4z-54≥0 means 14≤z≤24.

When z= 14, 15, it does not matter;

When z= 16, x=2, y = 6;

When z= 17, 18,19,20, it doesn't matter.

When z=2 1, x=6 is irrelevant.

A: The 25-question test is conducted twice. Comments: The key to solve this problem is to consider the values of x, y and z according to the characteristics of non-negative integers.