There was a beautiful japanese garden. within the garden, there is a pond. since the beginning of summer, lilies have started to cover more and more of it with every passing day. in fact, the quantity of lilies doubles every day. by day 58, the entire pond will be covered. how many days will it take the lilies to cover half the pond?

57 days

Step-by-step explanation:

Given that there was a beautiful Japanese garden.

Also given that within the garden, there is a pond.

Since the beginning of summer, lilies have started to cover more and more of it with every passing day.

In fact, the quantity of lilies doubles every day.

By day 58, the entire pond will be covered.

Since given that every day lilies double, we find that if on 58th day it was full, then exactly one day before it should be 1/2 of pond.  In that case only, on 58th day pond can become full.

Hence half pond is filled with lilies in 57 days.