A container holds 40 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 40 litres is removed, so 32/40 of the milk stays. After the first time: 40 × 32/40 = 32 litres of milk. After the second time: 32 × 32/40 = 25.6 litres. Formula: milk left = V × (1 − x/V)ⁿ for n repetitions. Subtracting 8 twice (24 litres) is wrong because the second removal takes out some water too.