A spider is trying to build a web for itself. The web built by it doubles every day.
If the spider completely built the web in 15 days, how many days did it take for the spider to build 25% of the web?
It will take 13 days for the spider to build 25% of the web.
Since the spider doubles the work done everyday, it would have built 50% of the web on day 14 and 25% of the web on day 13.