The naive answer is just the average of the speeds: (20 + 30)/2 = 25 nautical miles per hour. Perhaps surprisingly, this turns out to be the wrong answer: The ship will travel a longer time at 20 nautical miles per hour on the way to Hawaii than at 30 nautical miles per hour on the way back (because it is then going faster); therefore the average speed should be closer to the slower speed than to the faster speed: less than 25 nautical miles per hour.

The right way to look at the problem is to consider a stretch of 60 nautical miles: it took 3 hours to cross it from east to west at 20 nautical miles per hour, and then on the way back it took 2 more hours to cross it from west to east at 30 nautical miles per hour. This gives us a total of 120 nautical miles covered in 5 hours. What was the average speed? Would you get a different answer if the distance were something other than 60 nautical miles?