Water is building up in a bathtub. After 2 minutes there are 12 gallons of water and after 4 minutes there are 20 gallons of water. What is the average rate at which water is entering the bathtub (assuming it is constant). Let v represent the number of gallons in the tub at anytime t (in minutes) since water has been entering from the start. Write a linear equation for v in terms of t.