Samantha begins her road trip with 30 gallons of gasoline in the tank for a van or a van gets 25 miles to the gallon Y represent the number of gallons of gasoline in the tank after x miles of travel

y = - 1/25x + 30

The + 30 represents the 30 gallons of gasoline at the beginning of Samantha's road trip.

As each mile, x, does by, 1/25 of a gallon is lost (because the van gets 25 miles per gallon). so you put a negative 1/25 in front of the x. Therefore, when Samantha has driven x miles, she will have y gallons of gas left.