The value of a used car can be modeled by the formula v=vo(1-r)^t where vo is the car's purchase price, in dollars; r is the car's constant annual rate of decrease in value, expressed as a decimal; and v is the car's dollar value at the end of t years. a used car has a constant annual rate of decrease in value of 0.075. according to the model, what expression would give the number of years after purchase for the car to reach a value that is 50% of its purchase price?