Automobiles have different fuel economies ( mpg), and commuters drive different distances to work or school. Suppose that a state Department of ­Transportation ( DOT) is interested in measuring the average monthly fuel consumption of commuters in a certain city. The DOT might sample a group of commuters and ­collect information on the number of miles driven per day, number of driving days per month, and the fuel ­economy of their cars. Develop a predictive model for calculating the amount of gasoline consumed, using the following symbols for the data.

G = gallons of fuel consumed per month
m = miles driven per day to and from work or school
d = number of driving days per month
f = fuel economy in miles per gallon

Suppose that a commuter drives 30 miles round trip to work 20 days each month and achieves a fuel economy of 34 mpg. How many gallons of gasoline are used?