$12\mathrm{.}13$ In Fig. $12-12,$ pipe $1$ is $36-$in smooth concrete, $5000$ ft long; pipe $2$ is $24-$in cast iron, $1500$ ft long; and pipe $3$ is $18-in$ cast iron, $4000$ ft long. The elevations of water surfaces in reservoirs A and $B$ are $300$ ft and $250$ ft $,$ respectively, and discharge $Q_1$ is $50f\frac{t^3}{s}.$ Find the elevation of the surface of reservoir $C.$

Fig. $12-12$