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.
Note:
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.