From Mechanics

In this case \displaystyle {{R}_{B}}=0. Taking moments about A:

\displaystyle \begin{align} & 0\textrm{.}5\times 20\times 9\textrm{.}8=1\times m\times 9\textrm{.}8 \\ & m=\frac{0\textrm{.}5\times 20\times 9\textrm{.}8}{1\times 9\textrm{.}8}=10\text{ kg} \\ \end{align}