A class of 40 students has 25 boys and the remaining students are girls. What is the probability that a girl student would be chosen at random for a dancing competition?

The probability of an event is given as:

Probability of an event = $\frac{\mathrm{Outcomes} \mathrm{of} \mathrm{an} \mathrm{event}}{\mathrm{Total} \mathrm{possible} \mathrm{outcomes}}$

Total number of students= 40
The number of boys in the class= 25
The number of girls in the class = 40-25 = 15

The probability of selecting a girl for the dance competition  is :
=$\frac{\mathrm{Number} \mathrm{of} \mathrm{girls} \mathrm{in} \mathrm{the} \mathrm{class}}{\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{students} \mathrm{in} \mathrm{the} \mathrm{class}}$

$\begin{array}{rcl} & =& \frac{15}{40}=\frac{3}{8}\end{array}$

Therefore,$\frac{3}{8}$ is the correct response.