A container holds 50 litres of milk. 10 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, 10 of the 50 litres is removed, so 40/50 of the milk stays. After the first time: 50 × 40/50 = 40 litres of milk. After the second time: 40 × 40/50 = 32 litres. Formula: milk left = V × (1 − x/V)ⁿ for n repetitions. Subtracting 10 twice (30 litres) is wrong because the second removal takes out some water too.