Solution: There are X people in the second prize and (x+2 1) people in the third prize, so the total number of winners is 3x.

From the meaning of the question: 3x-x-(x+21) =1/5× [x+(x+21)]-3.

x-2 1 = 1/5×[2x+2 1]-3

x-2 1=(2/5)x+2 1/5-3

x-2 1=(2/5)x+6/5

3/5x= 1 1 1/5

x= 1 1 1/5×5/3

x=37

Then 3x= 1 1 1

So * * *11/classmates won the prize.