A tank contains $80$ gal of pure water. A brine solution with $2b\frac{l}{g}al$ salt enters at $2gpm,$ and the well$-$stired mixture leaves at the

same rate. How much time must pass belore the amount of salt in the lank reaches $80\%$ of the limling value?

a$.60$min

b$.62$min

c$.64$min

d$.65$min