Explanation:

To have final mixture with water 15% means, milk = (100-15) = 85%
Pure milk is 100% milk, and in 75 litres of milk there is 80% milk so
Milk (75 litres)                        Pure milk (x litres)
80                                                  100
.                             85
15                                                    5
3 : 1
So 75/x = 3/1
Solve, x = 25