A patch of moss doubles in size every day. On day 20 it covers a whole rock. On which day did it cover exactly half the rock?
Show hint
Go backward one day from the full rock.
Show answer
Day 19. It doubles every day, so it was half the size the day before.The moss doubles every day, so the day before it covered the whole rock it covered exactly half: day 19.
About this riddle
A doubling puzzle. Instead of counting forward from day 1, work backward from the day the rock is covered.
It comes from Math Brain Teasers, a collection of 25 riddles with hints and answers, sorted from easy to tricky. Play the video, pause it when the riddle ends, and see if you can solve it before the answer appears.
