$($B$).$ Design a rapid sand gravity filter to meet water demand of a town with population of $37500$ with average water dernand of $160$ lped $.$ The quantity of backwash water used is $\backslash (3\backslash \%\backslash )$ of filter output. The time lost in backwashing and cleaning of filter every day is $30$ minutes. The filter is backwashed at the rate of $\backslash (10\backslash$mathrm$\{$l$\}/\backslash$mathrm$\{$m$\}^\{2\}/\backslash$mathrm$\{$sec$\}\backslash )$ for $10$ minutes $.$ Assume suitable design data wherever required.