Mathematical method: three years: (804-36 )÷2=384 (person); Two years: 384+36=420 (person); Four years: 384-55=329 (person)

Equation method:

Solution: Suppose there are X students in grade three, then there are x+36 students in grade two.

x+x+36=804

2x=804-36

x=384

Number of students in Grade Two: 384+36=420; Number of students in Grade Four: 384-55=329.