A sum of $\text{Rs. }312$ was divided among $100$ boys and girls in such a way that each boy gets $\text{Rs. }3.60$ and each girl $\text{Rs. }2.40$. The number of girls is:

Let the number of boys be $b$ and the number of girls be $g$.
Given that total number of students is $100$:

$$b+g=100⟹b=100-g$$

The total amount distributed is $\text{Rs. }312$:

$$3.60b+2.40g=312$$

Substitute $b=100-g$ into the equation:

$$3.60(100-g)+2.40g=312$$

$$360-3.60g+2.40g=312$$

$$360-1.20g=312$$

$$1.20g=360-312$$

$$1.20g=48$$

$$g=\frac{48}{1.20}=40$$

Thus, the number of girls is $40$.