$6\mathrm{.}2$ A $375mm$ internal diameter, $1$:$258$ gradient sewer connects two manholes A and B$.$ The upstream manhole A has coordinates E $274\mathrm{.}698,$ N $842\mathrm{.}393,$ and the soffit level of the sewer leaving it is $16\mathrm{.}438m.$ Assuming negligible fall across manhole B $($E $342\mathrm{.}812,$ N $864\mathrm{.}844),$ what is the invert level of the $450mm$ diameter exiting pipe? If the cover level at manhole $B$ is $18\mathrm{.}590m,$ what is its depth?

$15\mathrm{.}710m,2\mathrm{.}880m$

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