A container holds 100 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 100 litres is removed, so 90/100 of the milk stays. After the first time: 100 × 90/100 = 90 litres of milk. After the second time: 90 × 90/100 = 81 litres. Formula: milk left = V × (1 − x/V)ⁿ for n repetitions. Subtracting 10 twice (80 litres) is wrong because the second removal takes out some water too.