The proportion of the number of students in three classes is 1 : 2 : 3. If 20 students are included in each class, then the proportion becomes 3 : 5 : 7. What was initially the total number of students in the three classes?

240

According to the question,

\(\frac{x + 20}{2x + 20}\) = \(\frac{3}{5}\)

⇒ 5x + 100 = 6x + 60

⇒ x = 40.

So, the total number of students initially

⇒ (1 + 2 + 3)x = 6x = 6 x 40 = 240