The relationship between class one and class three is an integer multiple, so let's put it aside for the time being. Class 3 is 20 more than Class 2, which means that Class 2 plus 20 is as much as Class 3. 180+20=200。 If the total number is 200, class one is twice as many as class three and class two, with four copies and class one with two copies. Class 3 and Class 2 each have one copy, so 200÷4=50, and one class has 100. Class three is 50, and class two is 50-20 = 30. If the equations are listed, it will be easy to solve.