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

Step 1: Understand the setup

• Total initial quantity of petrol = 200 liters
• Each time 40 liters is removed and replaced with kerosene
• This process is repeated 4 times

Step 2: Use the replacement formula

After n operations of removing x liters from total V liters and replacing with other liquid:

Remaining petrol = V × (1 - x/V)^n

Substitute values: V = 200, x = 40, n = 4

Remaining petrol = 200 × (1 - 40/200)^4
                 = 200 × (1 - 0.2)^4
                 = 200 × (0.8)^4
                 = 200 × 0.4096
                 = 81.92 liters

Step 3: Calculate the amount of kerosene

Total quantity in tank = 200 liters
Remaining petrol = 81.92 liters
Therefore, kerosene = 200 - 81.92 = 118.08 liters

Answer: 118.08 liters of kerosene

After replacing 40 liters 4 times, the total amount of kerosene in the tank becomes 118.08 liters.

This problem is a classic example of a successive replacement concept in quantitative aptitude. The key formula used here allows you to find how much of the original quantity remains after repeated removals and replacements. It's useful in problems involving dilution, adulteration, and real-life business scenarios.