hey guys, i think i might have to use dv/dt = -kv. i have no idea how do this problem, help me solve this

Suppose that a motorboat is moving at 40 ft/s when its motor suddenly quits, and that 10sec later the boat has slowed down to 20 ft/s. Assume that the resistance it encouters while coasting is proportional to the squire of its velocity. how far does the motorboat coast in the first minute after its motor quits?