I'm experiencing a mental block with this problem.. My professor said as a hint to find the general expression for P(Y=y). Thanks for your help.

A rental agency, which leases heavy equipment by the day, has found that one expensive piece of equipment is leased, on average, only one day in five. If rental on one day is independent of rental on any other day, find the probability distribution of Y, the number of days between a pair of rentals.