The density of a 3 molal solution of sodium thiosulphate $\left({\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3\cdot 5{\mathrm{H}}_2\mathrm{O}\right)$ solution is $1.25\mathrm{g}{\mathrm{cm}}^{-3}$ . The percentage by mass of sodium thiosulphate is 

Molar mass of ${\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3\cdot 5{\mathrm{H}}_2\mathrm{O}$ is $M=(2\times 23+2\times 32+3\times 16+5\times 18)\mathrm{g}{\mathrm{mol}}^{-1}=248\mathrm{g}{\mathrm{mol}}^{-1}$

The mass of 3 mol of ${\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3\cdot 5{\mathrm{H}}_2\mathrm{O}$ is $m_2=\left(3\mathrm{mol}\right)\left(248\mathrm{g}{\mathrm{mol}}^{-1}\right)=744\mathrm{g}$

The mass of solution will be $m=m_1+m_2=1000\mathrm{g}+744\mathrm{g}=1744\mathrm{g}$

Mass percentage of sodium thiosulphate  $=\frac{m_2}{m}\times 100=\left(\frac{744\mathrm{g}}{1744\mathrm{g}}\right)\times 100=42.67\mathrm{\%}$.