Tap A fills 1/2 of the tank / hour.Tap B fills 1/3 of the tank / hour.Tap C empties 1/6 of the tank / hour.Altogether, they fill (1/2) + (1/3) - (1/6) = 2/3 of the tank, after 1 hour.In conclusion, having started all the 3 taps, the tank will be filled in (3/2) = 1.5 hours.