A container holds 90 litres of milk. 9 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, 9 of the 90 litres is removed, so 81/90 of the milk stays. After the first time: 90 × 81/90 = 81 litres of milk. After the second time: 81 × 81/90 = 72.9 litres. Formula: milk left = V × (1 − x/V)ⁿ for n repetitions. Subtracting 9 twice (72 litres) is wrong because the second removal takes out some water too.