$4\mathrm{.}1\mathrm{.}5.$ Determine the elevation of the upstream reservoir $(A)$ in Figure P$4\mathrm{.}1\mathrm{.}2$ if the downstream reservoir $(B)$ is at $750$ m $,$ the flow rate $($water at $20C)$ through the smooth concrete pipeline is $1\mathrm{.}2\frac{m^3}{sec},$ given the following:

pipes $1$ and $2$: $100$ m long