Three class strengths A: B: C is 4:5:6 if each class strength is increased by 15 then the ratio is 6:7:8 when what is the total strength before increasing?

The number of strength units increases by 2 when 15 is added. Each unit is worth 15/2 = 7.5.
The original strengths (4:5:6) are
  30:37.5:45