Speed is the rate of change of the distance with respect to time.
Then, to find the speed, divide the change in position of the ship over the time that it takes for the ship to travel that distance.
Notice that the ship has traveled a distance of 37.5 miles after 1.5 hours, and that it has traveled a distance of 25 miles after 1 hour.
Since 37.5 - 25 = 12.5 and 1.5 - 1 = 0.5, then, the ship traveled a distance of 12.5 miles on 0.5 hours.
Divide 12.5 miles over 0.5 hours to find the speed of the ship:
Thereore, the speed of the ship is 25 miles per hour.