The family trip is a linear model and its equation can be written as a linear equation y = mx + b where m is the slope and b is the y-intercept. The slope is the constant rate of change. The rate of change here is 1 gallon per 20 miles or -1/20 since each gallon used decreases from the overall gas tank. Substitute m = -1/20.
The y-intercept is the starting point or the number of gallons the tank in the van holds. Since the van had 16 gallons after 120 miles, this means there was 22 gallons. Since 120 miles has been spent at 20 miles a gallon, it took 120 / 20 = 6 gallons to drive that far. So 16 + 6 = 22 gallons. Substitute b = 22.
