The number of bacteria in a container doubles every minute. If the container gets completely filled with bacteria in 30 minutes, then in how many minutes will one-fourth of the container be filled with bacteria?

The quarter mark is reached at 28 minutes.

The bacteria double every minute, so the amount halves going back one minute.

The container is full at 30 minutes, so half is filled at 29 minutes.

One-fourth is filled one minute before that, at 28 minutes.

Working backward by halving gives 1/4 at minute 28.