I have a car. Each new wheel can go upto 10000 km. I want to travel 7500 km to a tourist place in your car with my friend and come back to your place. For that I wanted to buy 4 extra wheels. But it was not possible to take 4 wheels in the car. After thinking some time, my friend proposed to take only two wheels. Can you explain how we travelled the 15000km with only 2 extra wheels?