$11\mathrm{.}4.$ Crazy Joe operates a canoe rental service on the Gualala River. He currently leases $15$ canoes from a dealer in a nearby

city at a cost of $$10$ per day per canoe. On weekends, when the water is high, he picks up the canoes and drives to a

launching point on the river, where he rents canoes to white$-$water enthusiasts for $$30$ per day. Lately, canoeists have

complained about the unavailability of canoes, so Crazy Joe has recorded the demand for canoes and found the

experience below for the past $20$ days:

Recommend an appropriate number of canoes to lease.