Question

Assume that water lilies double in area every 24 hours. In the spring, there is one water lily on a lake. After thirty days, the lake is completely (100%) covered with water lilies. How many days did it take for lilies to cover 25% of the lake?

Answer 1

now: 1 lily
after 1 day: 2 lilies = 2^1
after 2 days: 4 lilies = 2^2
...
after 30 days : 2^30
this is 100%, so 25% would be 2^30/4 = 2^28

2^28 = (2)^t
t = 28

or simply work backwards....
in 30 days it was 100% covered
in 29 days it was 50% covered
in 28 days it was 25% covered.

Answer 2

Are you sure this is not the question from the pioneer woman's quiz for an ipad??