Given:
Initially quantity of the solution = 10 litres
Milk in the initial solution = 8 litres
2 litres of pure milk is added replacing 2 litres of the mixture.
Above process is repeated two more times.
Calculation:
First Replacement:
Volume removed contains = 8/10 milk and 2/10 water.
Milk removed = 2(8/10) = 1.6 litres
Water removed = 2(2/10) = 0.4 litres
Milk left = ( 8 - 1.6 = 6.4 ) litres
Water left = ( 2 - 0.4 = 1.6 ) litres
Then, 2 litres of milk is added,
New Milk Quantity = ( 6.4 + 2 = 8.4 ) litres
Water remains the same at 1.6 litres.
Second Replacement:
Milk removed = 2(8.4/10) = 1.68 litres
Water removed = 2(1.6/10) = 0.32 litres
Milk left = ( 8.4 - 1.68 = 6.72 ) litres
Water left = ( 1.6 - 0.32 = 1.28 ) litres
Then, 2 litres of milk is added,
New Milk Quantity = ( 6.72 + 2 = 8.72 ) litres
Water remains the same at 1.28 litres.
Third Replacement
Milk removed = 2(8.72/10) = 1.744 litres
Water removed = 2(1.28/10) = 0.256 litres
Milk left = ( 8.72 - 1.744 = 6.976 ) litres
Water left = ( 1.28 - 0.256 = 1.024 ) litres
Then, 2 litres of milk is added,
New Milk Quantity = ( 6.976 + 2 = 8.976 ) litres
∴ 8.976 litres of milk is present in the mixture.

Shortcut Trick
Initially ratio of milk and water = 8 : 2 = 4 : 1
In 2 litres of mixture,
Quantity of water = 2 × 1/5 = 0.4 litre
Water after 1st replacement/Initial water = 0.4/2 = 1/5
| Initial water quantity (unit) | Final water quantity (unit) |
|---|
| 1st Replacement | 5 | (5 - 1) = 4 |
| 2nd Replacement | 5 | (5 - 1) = 4 |
| 3rd Replacement | 5 | (5 - 1) = 4 |
| Total | 5 × 5 × 5 = 125 | 4 × 4 × 4 = 64 |
⇒ 125 units = 2 litres
⇒ 1 unit = 2/125 litres
⇒ 64 units = 2/125 × 64 = 1.024 litres
So, Final milk quantity = 10 - 1.024 = 8.976 litres
∴ 8.976 litres of milk is present in the mixture.