$500$ metric ton per hour $($mtph$)$ of dry solid at feed solid content of $20\%$ is treated in a

flotation bank of $15$ cells. Each flotation cell has a capacity of ${500}^{f}t3$ and operates with a

flotation pulp having $15\%$ air. Assuming that the solid specific gravity is $3\mathrm{.}9,$ determine

the retention time available for the flotation feed in the given flotation bank.