For this case we have that the original price is $ 640. If this week is at $ 480 then the discount is:
$ 640- $ 480 = $ 160
Thus, the discount is $ 160.
We look for the percentage by making a rule of three:
$ 640 -------------> 100%
$ 160 -------------> x
Where "x" represents the discount percentage:

Thus, the discount percentage is 25%
Answer:
25%