From a tank of petrol which contains 200 litres of petrol the seller replaces each time with kerosen......

From a tank of petrol, which contains 200 litres of petrol, the seller replaces each time with kerosene when he sells 40 litres of petrol(or its mixture). Everytime he sells out only 40 litres of petrol(pure or impure). After replacing the petrol with kerosen 4th time, the total amount of kerosene in the mixture is 

The amount of petrol left after 4 operations  

= 200 × (1−40/200)⁴

= 200 × (4/5)⁴

= 200 × (256/625)

By: Amit Kumar