Fins, a beautiful little fish, was lost in the belly of a whale, exactly 4 meters deep behind the mouth. Luckily it was a very brave fish and managed to swim forward advancing 2/3 meter every half hour. Finally, when it reached his mouth, the whale spit it out immediately!
How long had it been?
In an hour and a half Fins Go forward 2 meters.
To travel 4 meters you will need 2 more times, or 3 hours in total.