Let B = # of boys;  G = # of girls.


From the condition, we have these two equations

B + G = 96,    (1)

G =       (2)


Substitute (2) into eq(1), replacing G. You will get

B +  = 96.


Multiply both sides by 5.  You will get

5B + 3B = 5*96  ====>  8B = 5*96  ====>  B =  = 5*12 = 60.


Answer.  There are 60 boys and 96-60 = 36 girls in the class.