**Answer:** 10 minutes.

In 24 hours there are 1,440 minutes.

So, in 24 hours the cold tap can fill 1440 ÷ 18 = 80 baths.

Also, in 24 hours the hot tap can fill 1440 ÷ 9 = 160 baths.

Finally, in 24 hours the plug hole can empty 1440 ÷ 15 = 96 baths.

So, in 24 hours we have 80 + 160 **-** 96 = 144 baths. So each bath would take 1440 ÷ 144 = 10 minutes to fill.