There was a frog that fell down inside a well. The well is 30 meters deep. Every day, the frog goes up 3 meters then goes down 2 meters. The question is this: how many days does it take the frog to get out of the well completely?
27
You probably thought 30. Actually it is 27, because on the 27th day, when the frog, like always, goes 3 meters up, it will arrive to the 30th meter of the well and get out, without the need of getting again 2 meters down.