Two puzzles for the price of one this week. The first can be found elsewhere on the internet, and requires a little thought; the second is an extension of the first, and is much trickier.
There is a wall nine metres high, and at the bottom of it there is a snail. The snail climbs for twelve hours, but is then tired and rests for twelve hours, before resuming its climb. In each twelve-hour climb it ascends three metres, and in each twelve-hour rest it slips back two metres. So every day it climbs three metres and slips back two.
How many days does it take for the snail to reach the top of the wall?
Once you have thought about that we can move on to the related (and harder) puzzle.
In this puzzle the same snail, having reached the top of the wall, wants to return to the bottom. How many days does it take to get back to the bottom?
Be warned – this is a much subtler problem.
Answers at 9.00 on Monday