From Mechanics

In this case the reaction at the left hand support will be zero. Taking moments about the right hand support:

\displaystyle \begin{align} & 1\times 18\times 9\textrm{.}8=2\times m\times 9\textrm{.}8 \\ & m=\frac{1\times 18\times 9\textrm{.}8}{2\times 9\textrm{.}8}=9\text{ kg} \\ \end{align}