You haven't shown us his answer or any of his work, so we don't know whether it was right or wrong, and we have no way to find his mistake if his work was wrong.
(10 discs) / (5.49 dollars) = (10/5.49) disc/dollar = 1.821 disc per dollar
(5.49 dollars) / (10 discs) = (5.49/10) dollar/disc = 0.549 dollar per disc
