Problem Description: A spider is trying to build a web for itself. It doubles the work done everyday.

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?

Solution:

Easier way to think about this problem is that on the previous day, half the work was done (day 14) so on day 13 half of half of the work was done, or to say, 25%