A 45-gallon tank is filled with water, and then water begins draining from the tank at a constant rate of 1.5 gallons per minute.
Let t represent the number of minutes since the tank started draining.
Let v represent the number of gallons of water in the tank.
Write a formula expressing v in terms of t.

v is a function of t, and we will call this function f (so that f(t)=v). Write a formula for f.