To balance the ruler with minimum weight of ' $\mathit{W}$ ' gf, the weight must be suspended from $100\mathrm{cm}$ mark.
In equilibrium,
Sum of anticlockwise moment $=$ sum of clockwise moment.
$\Rightarrow 50\times 10\mathrm{gf}\mathrm{cm}=\mathrm{W}\times 50\mathrm{gf}\mathrm{cm}$
$\therefore \mathrm{W}=\frac{500}{50}=10\mathrm{gf}$