A dishonest person wants to make a profit on the selling of milk. He would like to mix water (costing nothing) with milk costing 33 $ per litre so as to make a profit of 60% on cost when he sells the resulting milk and water mixture for 36$. In what ratio should he mix the water and milk?
Choose the correct choice from the following options
A) 7 : 15 B) 7 : 10 C) 7 : 8 D) 7 : 4 E) 7 : 2

Rationale: "first of all , let ' s consider 1 liter of the stuff he is going to sell - - - naive customers think it ' s pure milk , but we know it ' s some milk - water mixture . he is going to sell this liter of milk - water for $ 36 . this $ 36 should be a 60 % increase over cost . here , we need to think about percentage increases as multipliers . using multipliers ( cost ) * 1.60 = $ 36 cost = 36 / 1.6 = $ 22.5 if he wants a 60 % increase over cost on the sale of one liter of his milk - water , the cost has to be $ 22.5 well , a liter of milk costs $ 33 , so if he is going to use just $ 30 of milk in his mixture , that ' s 22.5 / 33 = 15 / 22 of a liter . if milk is 15 / 22 of the liter , then water is 7 / 22 of the liter , and the ratio of water to milk is 7 : 15 . answer choice ( a )"
Answer: A