On selling 17 balls at Rs. 720, there is a loss equal to the cost price of 5 balls. The cost price of a ball is:

$$\eqalign{ & {\text{(C}}{\text{.P}}{\text{.}}\,{\text{of}}\,{\text{17}}\,{\text{balls) - (S}}{\text{.P}}{\text{.}}\,{\text{of}}\,{\text{17}}\,{\text{balls) = (C}}{\text{.P}}{\text{.}}\,{\text{of}}\,{\text{5}}\,{\text{balls)}} \cr & \Rightarrow {\text{C}}{\text{.P}}{\text{.}}\,{\text{of}}\,{\text{12}}\,{\text{balls = S}}{\text{.P}}{\text{.}}\,{\text{of}}\,{\text{17}}\,{\text{balls = Rs}}{\text{.720}} \cr & \Rightarrow {\text{C}}{\text{.P}}{\text{.}}\,{\text{of}}\,{\text{1}}\,{\text{ball}} = Rs.\,\left( {\frac{{720}}{{12}}} \right) = Rs.\,60 \cr} $$