TWO BIKES
Lock up the kayaks at the starting point and drive car (with bikes) to the ending point. Ride bikes to starting point and lock them up. Kayak down the river to ending point. Put kayaks on the car and drive to starting point to pick up the bikes.
Lock up the bikes at the ending point. Drive the kayaks to the starting point. Kayak down the river to ending point, and lock up the kayaks. Ride the bikes to the starting point, and put them on the car. Drive to the ending point and pick up the kayaks.
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ONE BIKE
Drop the kayaks at the starting point where person 1 stays. Person 2 drives the car (with bike) to the ending point. Person 2 bikes back to the starting point and locks up the bike. Kayak down the river to ending point. Put kayaks on the car and drive to starting point to pick up the bike.
Lock up a bike at the ending point. Drive the kayaks to the starting point. Kayak down the river to ending point, and person 1 stays with the kayaks. Person 2 rides bike to the starting point, and puts it on the car. Person 2 drives to the ending point and retrieves person 1 and the kayaks.