A continuously stirred tank initially contains $300kg$ of a solution with a concentration of $6\mathrm{.}67\%$ salt. A constant flow rate of $700k\frac{g}{h}$ of the salt solution containing $13\mathrm{.}33\%$ salt is suddenly introduced into the tank, and a constant withdrawal rate of $400k\frac{g}{h}$ is also started. These two flows remain constant thereafter. Determine the time it will take to reach a concentration of $12\mathrm{.}54\%$ in the vessel.