A container holds 80 litres of milk. 8 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, 8 of the 80 litres is removed, so 72/80 of the milk stays. After the first time: 80 × 72/80 = 72 litres of milk. After the second time: 72 × 72/80 = 64.8 litres. Formula: milk left = V × (1 − x/V)ⁿ for n repetitions. Subtracting 8 twice (64 litres) is wrong because the second removal takes out some water too.