Assuming that the ship travels at a constant speed how far will the ship travel in 3.5 hours

Speed (s) = distance (d) divided by time (t)
You can see that every 30 minutes or 0.5 hours, the ship's distance increases by 12.5 miles. The problem even say the ship's speed is constant...so it doesn't matter which numbers you use for your calculation just used the time with the corresponding distance.
s = d / t
Use the 1st set of number t = 0.5 and d = 12.5
s = 12.5(miles) / 05(hours)
s = 25 miles per hour (miles/hour OR mph)
Check using the last set of numbers t = 1.5 and d = 37.5
s = 37.5 / 1.5
s = 25 miles per hour (mph)