**What was the task?**

We need 4 such cars to deliver the letter across the desert successfully.

Let's divide the entire route into

**6 parts.**That means the distance that car can travel (half the total path in desert) is divided into

**3 parts.**To travel each part car requires

**1/3rd**of it's petrol in the tank.

**1.**At first

**1/6th**of total path, all cars are

**2/3rd**full. Now 2/3rd of the petrol from 1 car can be used to fill 1/3rd of tanks in other 2 cars (1/3 + 1/3 = 2/3). This way, we would have

**2**cars

**full**while

**1**car

**2/3rd**full. We are leaving behind the empty car, taking 3 cars forward.

Stage 1 |

**2.**At next

**1/6th**of the distance,

**2**full cars will use

**1/3rd**of their petrol hence would be

**2/3rd**full. And the car that was

**2/3rd**at previous stage would be not

**1/3rd**full. At this stage, the petrol from car that is

**1/3rd**full can be used to fill tank of 1 car completely. So we are leaving behind one another

**empty**car here & taking

**fully filled car**&

**2/3rd**filled car for next stage.

Stage 2 |

**3.**For next

**1/6th**of the total distance, the car that was

**fully**filled would have

**2/3rd**petrol. And the car which was

**2/3rd**at previous stage would be now

**1/3rd**filled. The petrol of this car can be used to fill the tank of the first car. Now we have

**1 car fully**filled while other one is

**empty.**So we can leave behind the empty car & use fully filled car for the rest half of the journey. Remember, a car which tank is full can travel half the total path.

Stage 3 |

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