Question

The energy efficiency of a car is approximately 20%. If you put 10 gallons of gas into the car, how many gallons are “wasted”?

Answer 1

First, you find what 20% of 10 gallons of gas would be. This will show how many gallons the car actually uses.
10 gallons x 20% =
10 x 0.20 =
2 gallons used

Then you subtract that number from the total 10 gallons to get how many gallons of gas would be wasted.
10 gallons - 2 gallons = 
8 gallons of gas wasted