-- Water pouring into a tub with the drain closed
fills 1/10 of the tub each minute.
-- Water draining from a tub with the faucets off
empties 1/12 of the tub each minute.
When the faucet is filling AND the drain is draining, the tub collects
(1/10) - (1/12) of a tub each minute.
(1/10) - (1/12) = (6/60 - 5/60) = 1/60 of a tub per minute.
Filling (1/60) (tub/minute) corresponds to the unit rate of (60/1) (minute/tub).
