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.