Volume of bath tub in gallons (V) = 60
Rate of draining water in gallons per minute (R) = 12
When time t = t min
Volume of water drained out in gallons = R*t
Volume of water drained out in gallons = 12t
Water retaining in bath tub = Total volume - volume drained
Water retaining in bath tub = V - Rt
Water retaining in bath tub = 60-12t
When the amount of water retaining in bathtub is 0 then the tub is fully drained.
60-12t = 0
t = 5
So bathtub will be fully drained in 5minutes.
Here we have to assume that the draining rate of bath tub remains constant at all time. Usually it decreases when the water volume decrease.