Hi Anirudh Sriram,
Quantity of milk left after 1st iteration = 30 * 20/30​
Similarly, after 2nd iteration, Quantity = 30 * 20/30 * 21/30
We multiplied the milk after 1 iteration with 21/30 just because 9 litres of the mixture was removed and filled with water. So we are left with 30-9=21 .
I hope this helps!

here given aruns age is 40℅of barun

then how we get 2.5A

Hi, Age of Arun = 40% of age of Barun.
Let the age of Arun = x
=> x = 40% of age of Barun.
=> 0.4*(Age of Barun) = x
=> Age of Barun = x/0.4 = 2.5x

Hi Aquif, if A is Arun's age and B is Barun's age then, A = 40% of B. So B = A/40% = 100/40 * A = 2.5A

Hi Kashish Bhatter,
Yes, you are correct. We have to find the value of k/b (percent of change in years). We got that as 0.2. So, 20% will be answer.
I hope this helps!!