Solution:
Total milk = 60 litres
Drawn off = 12 litres
$$\frac{{{\text{Final quantity}}}}{{{\text{Initial quantity}}}}$$ $$ = {\left( {1 - \frac{x}{c}} \right)^t}$$
X = Replaced quantity
C = Capacity
T = Number of process
$$\eqalign{
& \frac{{{\text{Final quantity}}}}{{{\text{Initial quantity}}}} = {\left( {1 - \frac{{12}}{{60}}} \right)^2} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\left( {\frac{4}{5}} \right)^2} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{{16}}{{25}} \cr} $$
Ratio of milk and water in the resultant mixture :
=
16 : 9