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.
