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:

(C.P. of 17 balls) − (S.P. of 17 balls) = (C.P. of 5 balls)

⇒ C.P. of 12 balls = S.P. of 17 balls = Rs.720.

⇒ C.P. of 1 ball = Rs. $$\frac{720}{12}$$ = Rs. 60