A 30-litre mixture of milk and water contains 15% water. How much water must be added to it to increase the percentage of water in the solution to 37.5% ?

10.8 liters

Given,

Total mixture = 30L

Water contains = 15% of 30 L = 4.50 L,

Milk contains = 85% of 30 L = 25.50 L

Now we add some water, then water contains in the mixture is 37.5% of the total mixture.

The ratio of milk and water in the final mixture = 62.5 : 37.5 = 5 : 3

Let us add x liters of water, so

\(\frac{25.50}{4.50 + x}\) = \(\frac{5}{3}\)

76.50 = 22.50 + 5x

5x = 54

x = 10.8 L 

So the water added to the mixture is 10.8 liters.