Solution: Suppose there are X boys and x+28 girls in class one of six years.

Judging from the meaning of the question.

(x+28)*( 1- 1/4)+x=42

(x-28)*3/4+x=42

3x-84+4x= 168

7x=252

X=36 (person)

Participants: 36+(36+28)

=36+64

= 100 (person)

Number of students in the whole grade: 100÷20%=500 (persons)

There are 500 students in the whole grade.