A mixture contains milk and water in the ratio 3 : 5. If the total quantity is 40 litres, how much milk should be added to make the ratio 1 : 1?

The correct answer is 10 L.

Let's solve this step-by-step to find the required quantity of milk to be added.

Step 1: Calculate the initial quantities of milk and water.
The given total volume of the mixture is 40 litres.
The ratio of milk to water in the mixture is 3 : 5.
Sum of the ratio parts = 3 + 5 = 8 parts.

Quantity of Milk = $\frac{3}{8}\times 40$ = 15 litres.

Quantity of Water = $\frac{5}{8}\times 40$ = 25 litres.

Step 2: Determine the added quantity of milk to make the ratio 1 : 1.
A ratio of 1 : 1 means the quantity of milk must be equal to the quantity of water.
Since no water is added or removed, the quantity of water remains 25 litres.

To make the ratio 1 : 1, the total quantity of milk must also become 25 litres.

Required milk to be added = Target milk quantity - Initial milk quantity
= 25 litres - 15 litres = 10 litres.

Therefore, 10 L of milk should be added.