A large container is kept in open under the rain. Every passing hour, the water collected inside the container becomes double it was.

In ten hours, the container is filled completely. Can you calculate how long would it have taken to be filled half?

The answer is 9 hours.

Since we know that the water becomes double in the quantity every passing hour and that it was filled completely in ten hours, we need to go back one-step. Take out one hour and divide the quantity by 2.