Lee wanted to fill an empty swimming pool with water. He turned on the tap at 13:50. At 14:20, the swimming pool was 1/4–full of water. If he wanted it to be 3/4 full of water, at what time should he turn off the tap? Give your answer in a 24-hour clock format, such as 19:00.

1/4–full of water: 14:20-13:50 = 30 min

3/4–full of water: 30 min × 3 = 1 h 30 min

Start time: 13:50

End time: 13:50 + 1 h 30 min = 15:20

So the answer is 15:20.