An outlet pipe can empty a cistern in 4 hours. In what time will it empty 3/4 part of the cistern?

Question: An outlet pipe can empty a cistern in 4 hours. In what time will it empty 3/4 part of the cistern?

Solution:
The outlet pipe empties one complete cistern in 4 hours
Time taken to empty 3/4 Part of the cistern = (3/4) × 4 = 3 hours.