The distance each travels is proportional to its speed. The given speeds are at right angles to each other in a 5:12 ratio. The Pythagorean theorem tells you the hypotenuse of such a right triangle will have length ...
√(5²+12²)=√169 = 13
That is the speeds are in the ratio 5:12:13, where 13 represents the relative separation speed. Actual speeds are then 50 mph : 120 mph : 130 mph.
So the distance between the boat and steamer will be increasing at 130 mph. The distance will be 130 miles after 1 hour.
