Question

The amount of water flowing into a tank doubles every minute. The tank if full in an hour. When is the tank half full.

Answer 1

Since the water in the tank is doubling each minute, that means the minute before the tank was full, it was half full.
If it fills in an hour, which is 60 minutes, than that is double what it was 1 minute ago.

The invert of doubling is halving.
The tank was half full a minute before it was full.
It was full after 60 minutes.
This means it was half full after 59 minutes.

Answer:
The tank was half full after 59 minutes.

Hope this helps!