$\backslash$table$[[$Sewage Collection System $-,$scale $1$:$2000],[$Proposed Subdivision Development,$],[$scale $1$:$2000,]]$

b$.$ a$.$ An urban house is to be connected to an existing sewer system with a branch line invert level of $82\mathrm{.}5m(300mm$ pipe$).$ It is proposed that the sewer house drain will be a $100mm$ diameter pipe, with a $300mm$ allowance for the house drain connection and provision for $300mm$ of soil pipe cover. Calculate the house drain grade if the minimum surface RL of the house is $83\mathrm{.}6m$ and the house pipe outlet is $16m$ from the branch line. $(25$ marks$)$