A school has three sections whose average marks are 60, 70 and 80. The number of students in the first two sections is in the ratio $1 : 2$. The overall average of all three sections is 72. If the first section has 25 students, how many students are in the third section?

Explanation

1. 01

   Let the sections have 1, 2 and x parts of students.

2. 02

   Overall average: $\frac{60 + 2 \times 70 + 80x}{3 + x} = 72$.

3. 03

   $200 + 80x = 216 + 72x$, so $8x = 16$ and $x = 2$.

4. 04

   Ratio $1 : 2 : 2$; with 25 in the first section, the third has 50.