Question

Lucas bought a used car for x dollars. one year later the value of the car was 0.80 x. which expression is another way to describe the change in the value of the car

Answer 1

Knowing that a year later, the car is only worth 0.80 x, you can simplify that by stating that one year after purchase, the car is only worth 80% of its original purchase price.   
Another way of stating that is that the car lost 20% of its value of the course of one year.