A mixture of milk and water contains 75% milk. How many litres of water must be added to 60 litres of this mixture to raise the percentage of water to 40%?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 15 litres
Adding water does not change the quantity of milk; only its percentage falls.
Milk \(=75\%\) of 60 \(=45\) litres
In the new mixture milk is \(100-40=60\%\), so the new total \(=45\div0.6=75\) litres
Water to add \(=75-60=15\) litres
15 litres of water must be added.
Why the other options are incorrect
Option (1) 9 litres — Incorrect. This is 15% (40% − 25%) of the old 60 litres; the new percentage is of the larger new total. Adding 9 litres gives 24 litres of water in 69 litres, which is not 40% water.
Option (3) 24 litres — Incorrect. This is 40% of the old 60 litres. Adding 24 litres gives 39 litres of water in 84 litres, which is not 40% water.
Option (4) 20 litres — Incorrect. Adding 20 litres gives 35 litres of water in 80 litres, which is not 40% water.