You drive a car that holds 15 gallons of gasoline. your car uses 0.03 gal per mi. which equation best represents this information?

Assuming you want to know how far you can travel before running out of gas.
.03n=15 will answer that question

Divide both sides of the equation by .03

.03n   = 15
.03       .03

You will be able to drive 500 miles.  This is often called the cruising range.  Most people avoid getting to close to the cruising range as they do not want to have to walk to a gas station when they run out and also because the last ounces of gas may have dirt and debris in them (which collected over time) which has settled to the bottom of the gas tank and will now go through their engine (possibly fouling it up) when they fully drain the tank.