My bath has two taps and a plug hole.
The cold tap on its own fills the bath in 18 minutes, the hot one in 15 minutes.
The plug hole can drain the bath in 10 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: 45 minutes.
In 24 hours:
the cold tap could fill 80 baths
the hot tap could fill 96 baths
the plug could drain 144 baths
Therefore, in 24 hours, we have 80 + 96 - 144 baths full = 32 baths.
Therefore each bath would take 45 minutes to fill.