In the figure BKT$-104$ the rectangular flashboard $MN$ shown in cross section $(a=5\mathrm{.}4m)$ is pivoted at $B.$

$1.$ What must be the maximum height of B above N if the flashboard is on the verge of tipping when the water surface rises to M$?$

$2.$ If the flashboard is pivoted at the location determined above and the water surface is $1$ m below M$,$ what is the horizontal reaction at B$?$

$3.$ What is the reaction at N$?$