My bath has two taps and a plug hole.
The cold tap on its own fills the bath in 12 minutes, the hot one in 8 minutes.
The plug hole can drain the bath in 24 minutes with the taps off.
How long will the bath take to fill if I leave both taps on with the plug left out?
Answer: 6 minutes.
In 24 hours, the cold tap could fill 120 baths, the hot tap could fill 180 baths and the plug could drain 60 baths.
Therefore, in 24 hours, we have 120 + 180 - 60 baths full = 240 baths.
Therefore each bath would take 6 minutes to fill.