A lily pad starts in the middle of a pond on day 1, overnight it doubles in size. This happens every night. If the pond is covered on day 10, how big was the lily pad on day 9?
Hint: Work backwards, this way you don't need to know the size of the lily pad at the start.
Answer: 1/2 the size of the pond.
This is easiest to see by working backwards. On day 10 the pond is covered, so during the previous night it doubled in size.