18 hl ------------------- 4 robinete -------------------- 360 min
18 hl ------------------- 1 robinet ---------------------- 360 : 4 = 90 min
1 hl ------------------- 1 robinet ---------------------- 90 : 18 = 5 min
12 hl ------------------- 1 robinet ---------------------- 12 x 5 = 60 min
12 hl ------------------- 6 robinete --------------------- ? min = 60 : 6 = 10 min