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

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

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No because a circle or pie graph usually shows percentages you'd need a line graph