A container holds 200 litres of milk. 20 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, 20 of the 200 litres is removed, so 180/200 of the milk stays. After the first time: 200 × 180/200 = 180 litres of milk. After the second time: 180 × 180/200 = 162 litres. Formula: milk left = V × (1 − x/V)ⁿ for n repetitions. Subtracting 20 twice (160 litres) is wrong because the second removal takes out some water too.