Pure milk costs 3.60 per litre. A milkman adds water to 25 litres of pure milk and sells the mixture at 3 per litre. How many litres of water does he add?  Pick one.
A) 2 litres B) 5 litres C) 7 litres D) 11 litres E) none of these

Rationale: in mixture , quantity of pure milk / quantity of water = 3 − 0 / 3.6 − 3 = 3 / 0.6 = 5 / 1 since in every 5 litres of milk , he adds 1 litre of water . ∴ in every 25 litres of milk , he adds 5 litres of water . answer b
Answer: B