Class A is twice the size of Class B, so Class A = 2 * size of Class B
If Class B has 20 people Class A has 20 * 2 = <<20*2=40>>40 people
Since Class A is also 1/3 the size of Class C then Class A = 1/3 * Class C
This means 40 = 1/3 * Class C
Thus the size of Class C is 3 * 40 = <<3*40=120>>120 people
#### 120