Question 14.

Let's assume that after n iteration, the volume of the milk will be less than 50%, which is less than 20 liters.

Initially, the amount of milk is 40 liters, after the first iteration, the volume of milk is $40 \cdot \frac{9}{10}$

After the second iteration, the volume of milk is $40 \times \left(\frac{9}{10}\right)^2$

Similarly, after the n iterations, the volume of milk is $40 \times \left(\frac{9}{10}\right)^n$

Now,

$40 \times \left(\frac{9}{10}\right)^n \leq 20$

=> $\left(\frac{9}{10}\right)^n \leq \frac{1}{2}$

=> $n \geq 7$

Hence, the correct answer is 7