You and a friend are going camping for the night and you both bring camping stoves. Your stove is 25 percent efficient, and uses 0.1 liters of propane fuel to boil 10 cups of water. Your friend's stove is 50 percent efficient. How much propane fuel will your friend need in order to boil 10 cups of water?

Answer - 0.05 lit of propane fuel will your friend need in order to boil 10 cups of water.

Solution-

The more efficient the stove is, less the fuel you need to burn.

so efficiency and amount of fuel needed are inversely proportional with each other.

Given

Your stove is 25 percent = 0.25 efficient, will boil 10 cups of water, using 0.1 lit of fuel.

0.25 * 2 = 0.5 = 50 percent efficient stove, will boil 10 cups of water, using 0.1/2 = 0.05 lit of fuel.

(∵ As multipling a factor of 2 with efficiency will make the amount of fuel needed half.)