Your bath has two taps. With the plug in, one will fill the bath in four minutes; the other will fill the bath in five minutes. The bath takes two and a half minutes to empty from full. With both taps ON and the plug OUT, how long will the bath take to fill?
In 20 minutes, Tap 1 can fill 5 baths and Tap 2 can fill 4 baths making 9 baths in all. However the plug is out and in this time we could empty 8 baths.