Hi Mrunal Deshpande,
$$F=I\left(1-\frac{K}{V}\right)^n$$
If a mixture contains solutions A and B, and if certain amount of solution is replaced with B for n times, then
F - Final solution of A in mixture
I - Initial solution of A in mixture
K - volume of solution replaced
V - Total volume of the solution
n - number of times the process is repeated
In this question, milk is A and water is B.
I = 500, K = 50, V = 500 and n = 3
Hope this helps!

Hi Mrunal Deshpande,
$$F=I\left(1-\frac{K}{V}\right)^n$$
If a mixture contains solutions A and B, and if certain amount of solution is replaced with B for n times, then
F - Final solution of A in mixture
I - Initial solution of A in mixture
K - volume of solution replaced
V - Total volume of the solution
n - number of times the process is repeated
In this question, milk is A and water is B.
I = 500, K = 50, V = 500 and n = 3
It should be $$\left(0.9\right)^3$$ instead of $$\left(0.5\right)^3$$
Hope this helps!

Hi Soumyadeepta, 
The quantity of milk that is not replaced is 450, and total quantity of milk is 500. 
mf = 450/500 = 0.9 
Hope this helps!