One lily pad, doubling in size every day, covers a pond in 30 days. How long would it take eight lily pads to cover the pond? 
A lily pad sits on a pond. It doubles in size every day. It takes 30
  days for it to cover the pond. If you start with 8 lily pads instead,
  how many days does it take to cover the pond?

I think that the answer is $27$, but I don't really think that makes sense intuitively. I think that, intuitively, the answer should be less than $30/4$ since it is increasing at an exponential rate.

Another way to look at it is to work backward.
First just consider the one lily pad. After $29$ days it covers half the pond. After $28$ days a quarter of the pond. After $27$ days an eighth of the pond.
So after $27$ days eight lily pads would cover the whole pond.
