The boys and girls in a college are in the ratio 3 : 2. If 20% of the boys and 25% of the girls are adults, the percentage of students who are not adults is -

Answer: Option C

Let the number of boys and girls be 3x and 2x respectively
Then,
Number of boys and girls who are adults
$$\begin{array}{rl}& =20\%\text{ of }3x+25\%\text{ of }2x\\ & =\left(\frac{20}{100}\times 3x\right)+\left(\frac{25}{100}\times 2x\right)\\ & =\frac{3}{5}x+\frac{x}{2}=\frac{11x}{10}\end{array}$$
∴ Number of boys and girls who are not adults
$$\begin{array}{rl}& =\left[\left(3x+2x\right)-\frac{11x}{10}\right]\\ & =5x-\frac{11x}{10}\\ & =\frac{39x}{10}\\ & \text{Required percentage}\\ & =\left(\frac{39x}{10}\times \frac{1}{5x}\times 100\right)\%\\ & =78\%\end{array}$$