Respuesta :

combine like terms
b + 1.5b + 3 + 15 + 2b - 9 = 90
4.5b + 9 = 90

solve
4.5b + 9 = 90
4.5b = 81
     b = 18

answer
first:            4.5b + 9 = 90
second:      18 students