KillerSquirrel wrote:

Water lily grows at such a rate that it covers double the surface that it covered the day before. It takes 30 days for a water lily to cover the entire surface of the pond. How long would it take for two water lilies of the same size to cover the entire surface of the same pond?

A) 8.33

B) 12.5

C) 15

D) 20

E) 29

I like this question. Thought to repost it for our newcomers.

Have Fun.

P.S - ywilfred , let them try before you solve this

Thanks

Logically I can determine the answer is 29. (One lily covers 1/2 on the 29th day. If there is one more lily it will also cover 1/2 on the 29th day. Therefore, the whole pool will be covered).

However, mathematically I cannot come up with a solution; I don't understand the above 2 solutions either. Can someone explain it mathematically?