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.