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.