Answer:
1 hour - how many more hours will the tank be out of gas
Explanation:
The path of the boat, the horizontal distance and vertical distance from the dock are in the shape of a right triangle
Distance of boat from dock after 1 hour
d =

Speed of boat = 10mph
distance that can be traveled with 4 gallons of fuel
d = 5 mpg x 4 g = 20 miles
The boat has already traveled 10miles in the diagonal direction. Assuming same trajectory, the boat can travel 20 - 10 = 10 miles more
10 miles ==> 1 hour more at 10 mph
So additional time the boat can travel = 1 hour
I hope that helps. Tried to do my best to cut down the words