The solubility of ${\mathrm{Na}}_2{\mathrm{SO}}_4$ given in $\mathrm{g}$ per $100\mathrm{g}$ of ${\mathrm{H}}_2\mathrm{O}$
Solution can be easily converted $\mathrm{g}$ per $\mathrm{kg}$. $18\mathrm{g}$ per $100\mathrm{g}$ means $18\mathrm{g}$ of the solute will be contained in $100\mathrm{g}$ of the solvent (water) or solution. Again, recall that $1\mathrm{kg}$ is equivalent to $1000\mathrm{g}$.
If $100\mathrm{g}$ contains $18\mathrm{g},1000\mathrm{g}(1\mathrm{kg})$ will contain $\frac{18\mathrm{g}}{100\mathrm{g}}\times 1000\mathrm{g}=180\mathrm{g}$ $1000\mathrm{g}(1\mathrm{kg})$ will thus contain $180\mathrm{g}$. Hence, the solubility can be quoted as $180\mathrm{g}$ per $\mathrm{kg}$