$\text{Rs. }6000$ is divided among $40$ boys and some girls so that each boy gets $\text{Rs. }100$ and each girl gets $\text{Rs. }250$, then the number of girls is:

Let the number of girls be $g$.
* Total money received by $40$ boys $=40\times 100=\text{Rs. }4000$
* Total money received by $g$ girls $=g\times 250=250g$

The sum of the money distributed is given as $\text{Rs. }6000$:

$$4000+250g=6000$$

$$250g=6000-4000$$

$$250g=2000$$

$$g=\frac{2000}{250}=8$$

Thus, the number of girls is $8$.