You are given a 3 liter gas can and a 5 liter gas can, which you can fill from a pump.
You need to use them to put exactly 4 liters of fuel in the tank of you motorbike. If you put less than 4 liters it won't be enough to cross the desert, and if you put more than 4 liters the bike will be too heavy, and you will sink in the desert sand.
You cannot reach the gas pump with your bike, so you have to get the right amount of fuel in the gas cans and walk.

Fill 3 liter can
Fill 5 liter can