On a roadtrip, the distance driven can be represented by the following equation
 y=15x
 where x is the number of gallons of gas.
 What does the 15 represent?
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 A) The road trip was 15 miles long.
 B) The car traveled 15 miles per gallon.
 C) The gas cost $15.
 D) 15 gallons of gas were used.