This is a famous problem; you may have seen it. It appeared in an arithmetic textbook published in Nuremberg in 1561; Stella thinks that this was the first time it appeared in print. Anyhow, the problem is this: A snail has fallen into a dry well 21 meters deep. It starts climbing up the side of the well so as to escape. Each day it climbs 7 meters, but each night it slides back 2 meters. If it starts climbing on July 1, on which day will it finally be out of the well?
Well, the quick answer starts with the calculation that the snail�s net gain is 5 meters a day. In four days it will have risen 20 meters, so on the fifth day it gets out.
But why would a problem this easy be so well known? Here�s why. After three days, the snail has risen 15 meters. On the fourth day, it can climb seven meters more, and that seven meter advance puts it at 21 and beyond, so it can get to the top and over the rim safely on the fourth day.