A container holds 120 litres of milk. 24 litres are taken out and replaced with water. This is done once more. How much milk is now in the container?

💡 Solution & Explanation

Each time, 24 of the 120 litres is removed, so 96/120 of the milk stays. After the first time: 120 × 96/120 = 96 litres of milk. After the second time: 96 × 96/120 = 76.8 litres. Formula: milk left = V × (1 − x/V)ⁿ for n repetitions. Subtracting 24 twice (72 litres) is wrong because the second removal takes out some water too.