A porous cylinder, \(2.5 \mathrm{~cm}\) in diameter and \(80 \mathrm{~cm}\) long, is saturated with alcohol and maintained in a stirred tank. The alcohol concentration at the surface of the cylinder is maintained at \(1 \%\). The concentration at the center is measured by careful sampling and is found to drop from \(30 \%\) to \(8 \%\) in \(10 \mathrm{~h}\). Find the center concentration after 15 and \(20 \mathrm{~h}\).