Answer :
60$. These are my equations :
600 * 0.2 = 120 ; 600 - 120 = 480
Then...
600 * 0.1 = 60 ; 600 - 60 = 540
Which then became....
540 - 480 = 60
600 * 0.2 = 120 ; 600 - 120 = 480
Then...
600 * 0.1 = 60 ; 600 - 60 = 540
Which then became....
540 - 480 = 60