13 robinete............72 ore..................4680 l
1 robinet ....... 72×13=936 ore.....4680 l
1 robinet ..........1 ora ............4680:936=5 l
9 robineti ..........1 ora .............5 ×9=45 l
9 robineti ..........6750:45=150 ora .............6750 l