Set the number of small classes as x, the number of middle classes as y and the number of large classes as z.

According to the question:

x+40=z

4x =x+y+z

y =z*3/4

Now, according to Formula 3, change Y in Formula 2 to z*3/4, that is, bring 3 into 2.

Then equation 2 equals 4x = x+z * 3/4+z.

3x = z*7/4

4.x=？ z*7/ 12

Substitute Equation 4 into Equation 1.

z*7/ 12 +40 = z

40= z*5/ 12

40* 12/5=z

z=96

Z=96 is brought into the formula 1: x=96-40=56.

Z=96 is brought into Formula 3: y=72.

Small Class 56 Middle Class 72 Large Class 96