In a reactor a small amount of undesirable by$-$product is continuously formed at the rate of $0\mathrm{.}5$ lb$/$hr when the reactor contains a steady inventory of $10,000$ lb of material. The reactor inlet stream contains traces of the by$-$product $(40$ ppm$),$ and the effluent stream is $1400$ lb$/$hr$.$ If on start$-$up$,$ the reactor contains $5000$ ppm of the by$-$product, what will be the concentration in parts per million at $10$ hr$?$