Mohit went to a shop to buy $50\text{kg}$ of rice. He buys two varieties of rice which cost him $\text{Rs. }4.50$ per kg and $\text{Rs. }5$ per kg. He spends a total of $\text{Rs. }240$. What was the quantity of less expensive rice bought by him?

Let the quantity of the less expensive rice ($\text{Rs. }4.50$ per kg) be $x\text{kg}$.
Since the total mass bought is $50\text{kg}$, the quantity of the more expensive rice ($\text{Rs. }5$ per kg) must be $(50-x)\text{kg}$.

The total cost equation is given as $\text{Rs. }240$:

$$4.50(x)+5(50-x)=240$$

$$4.5x+250-5x=240$$

$$-0.5x+250=240$$

$$-0.5x=240-250$$

$$-0.5x=-10$$

$$x=\frac{-10}{-0.5}=20\text{kg}$$

Thus, the quantity of less expensive rice bought by Mohit is $20\text{kg}$.