It is tempting to say 24 days because it is half of 48 days. However, the answer is actually 47 days. If the lily pad patch doubles in size every day then from half filling the lake to completely filling the lake will require a single doubling of size which will take 1 day. If you take 1 day off 48 days you will get 47 days..
How fast do the lily pads grow?

Today’s puzzle challenge is a variation on a common problem of replication. How fast do the lily pads grow? See if you can figure out the answer in under 1 minute.
A patch of lily pads float in a lake. 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 of the lake?