You run through the woods and come to a road where you hitchhike. A couple picks you up and tells you that the only safe place is an island in the middle of the lake because zombies are too slow to swim over. You join them on the boat. If the boat’s speed with the current is 15 km/hr and the speed of the current is 2.5 km/hr. If you’re going against the current, the boat’s speed is: