Half the Lake

In a lake, there is a patch of lily pads. Every day, the patch doubles in size.
If it takes 48 days for the patch to cover the entire lake, how long would it take for the patch to cover half the lake?

Answer: 47 days.

Explanation:
Because the patch doubles in size each day, the day before it covers the whole lake, it must have covered exactly half.
So if the lake is fully covered on day 48, it’s half covered on day 47.