$4..$ The gate in Fig. B is subjected to water pressure on one side and to air pressure on the other side. Determine the value of $\backslash ($ x $\backslash )$ for which the gate will rotate counterclockwise if the gate is $($a$)$ rectangular, $1\mathrm{.}5$ m by $1\mathrm{.}0$ m $,($b$)$ triangular, $1\mathrm{.}5$ m base and $1\mathrm{.}0$ m high.