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
The graph is misleading because it does not start out at 0 inches of rainfall. To make it accurate, you'd redraw it and start with 0 inches of rainfall and go up from there.
