The students in the three classes = 4 : 6 : 9 Students increased in each class = 12 Ratio change = 7 : 9 : 12 According to the statement, 4x+12:6x+12:9x+12 or a:b:c We can write it as a : b : : b : c. (4x+12):(6x+12)::(6x+12):(9x+12) Ratio change = 7 : 9 : : 9 : 12
(4x+12)(6x+12)
(6x+12)(9x+12)
=
7×9
9×12
(4x+12)
(9x+12)
=
7
12
x=4 Total number of students in three classes before the increase is =4x+6x+9x =4×4+6×4+9×4=76