Troy's truck has a 30 gallons gas tank and gets an average of 21 miles per gallon.

Write a linear equation to represent the amount of gas in Troy's truck after driving a certain number of miles (assuming he starts with a full tank). Define your variables.

We are given: Initial amount of the gas = 30 gallons.

Milage of the Troy's truck = 21 miles per gallon.

Number of gallons of gas used in 1 miles = 1/21 gallons.

Let us assume, number of miles Troy truck travel = m miles.

Number of gallons of gas used in travelling m miles = m * 1/21 = 1/21 m.

And number of gallons of gas left after travelling m miles = g gallons of gas.

Number of gallons of gas = Total gas present initially - Number of gallons gas used to travel m miles.

Therefore, we can setup an equation.

g = 30 - 1/21 m.