Maths problem? Please help.?

Question

a certain substance doubles its volume every minute. At 9.00 am. a small amount is placed in a container and at 10.00 am the container just fills. At what time was the container one eighth full?

Answer 1

10:00 _ The container is full.

9:59 _ The container is 50% full.

9:58 _ The container is 25% full.

9:57 _ The container is 12.5% full.

The container was 1/8th full at 9:57 AM.