Q. A milkman mixes 5 litres of mineral water with 20 litres of pure milk. The mixture is sold at a price per litre that is 20% more than the cost per litre of milk, and the milkman makes a profit of 40%. Find the ratio of the cost per litre of mineral water to that of milk.

A. \(\frac{2}{7}\)

B. \(\frac{3}{7}\)

C. \(\frac{6}{13}\)

D. \(\frac{4}{7}\)

Since selling rate is 20% i.e. 1/5 more than cost of milk, assume cost of milk to be Rs. 5/lt, and cost of mineral water to be Rs. x/lt.

Find total cost and total sales. And use the data that profit% is 40%.

A. \(\frac{2}{7}\)

Let’s assume C.P. of 1 lt pure milk = 5, and C.P. of 1 lt mineral water = x

⇒ S.P. of 1 lt of mixture = 6

Total volume sold = 20 (milk) + 5 (water) = 25 lt

Total C.P. = 20 × 5 + 5 × x i.e. 100 + 5x

Total S.P. = 25 × 6 = 150

Now, Profit = 40%

S.P. =  1.4 × C.P.

150 = 140 + 7x

7x = 10

x = \(\frac{10}{7}\)

Answer = \(\frac{\text{water/lt}}{\text{milk/lt}} = \frac{x}{5} = \frac{2}{7}\)