A 4 m telescope can collect a given amount of light in 3 hour. Under the same observing conditions, how much time would be required for a 8 m telescope to perform the same task?

The light collection of a telescope is proportional to the AREA of the objective lens or mirror. So the 8m collects 4 times as much light as the 4m does in the same time. Or the bigger one collects the same amount of light in 1/4 of the time that it takes the smaller one.
Under the same observing conditions that the 4m needs 3 hours, the 8m monster can do the job in 45 minutes.