Question 389028
Let x=rate per liter that he pays for milk and also the rate that he sells the milk mixture 
Let y=amount of liters of milk initially purchased
And let w=amount of water that needs to be added

Cost of milk =xy
Value of the milk mixture=x(y+w)
A 20% gain in outlay can be written as follows: (% gain =(new amt-old amt)/old amt)
(x(y+w)-xy)/xy=0.2
Notice that the cost is not relevant
((y+w)-y)/y=0.2 or
w/y=0.2=1/5
This tells us that for every six liters of mixture, 5 liters is milk and 1 liter is water

Hope this helps---ptaylor