4 robinete .............1 bazin.............5 ore
1 robinet...............1 bazin............4×5=20 ore
6 robinete .............1 bazin.............20:6=3,(3) ore
1 ora .....60 min
0,(3) ore....x min
x=0,(3)×60=3/9 ×60=1/3 ×60= 20 min
3,(3) ore = 3 ore 20 min