Formula Used: Gain = Selling price − Cost price Gain percentage =[( Selling price − Cost price )∕ Cost price ]×100 Calculation: Let the selling price of 1 litre milk be Rs. x then, selling price of 33 litres of milk =33x Also given that, Gain = Selling price of 11 litres =11x Gain = Selling price of 33 litres of milk - Cost price of 33 litres of milk ⇒11x=33x - Cost price of 33 litres of milk ⇒ Cost price of 33 litres of milk =33x−11x=22x Gain percentage =[( Selling price − Cost price )∕ Cost price ]×100 Gain percentage =[(33x−22x)∕22x]×100 ⇒(11∕22)×100=50% ∴ The gain percentage is 50%.