Suppose you've got a car. But, you can only take 4 jerrycans with you. And one full tank (which can hold exactly the contents of one jerrycan). The content of every jerrycan is enough for travelling 1km. At the start are $\infty$ jerrycans!
Now, there are a few questions and we couldn't find an answer for it. So can you help us with those questions?
With 20 jerrycans you can travel 10km if you want to travel back. Now, if you want to travel in one straight line without going back, you can travel 10km. What is the best strategie to travel more than 20km?
What is the longest distance you can travel without going back to the start?
What is the minimum number of jerrycans needed to travel more than 20km?