$(20)5.$ Problem $5\mathrm{.}2$

$5\mathrm{.}2.$ A river receives an input of $FC$ bacteria as shown.

Five miles downstream from the outfall a concentration of $185,\frac{000}{100}ml$ is measured.

$($a$)$ Determine the decay rate in the stream $($day ${?}^{-1}).$

$($b$)$ Determine the effluent percent reduction so that the FC bacterial count at $x=5$

$mi$ is reduced by a factor of $1000.$