If the class size is 1, that is 1, then there are 1 people who take part in labor, and there are 4 people who don't, accounting for1(1+4) =1/5 of the class size.

Two people are equivalent to1/5-1/6 =1/30 of the class size.

The number of students in this class is 2÷( 1/30)=60.