@StormCaster Initially I just put the values you had given without thinking much. Now when I think about it, how are you finding these values? Using the atomic mass of Sb as 122 g and sulphur as 32 g, we get the moles of $\ce{Sb_2S_3}$ as $\dfrac{1000}{122\times 2+ 32 \times 3}=2.941$ and moles of $\ce{H_2}$ as 10/2 =5. Now how will you find the equilibrium concentrations of these without knowing anything else? Are you sure you are not missing any other detail? May I know which book are you using?

@Ashish The question was asked in FIITJEE AITS, and this is the solution given by them, so I can't say it is from a verified source. Although it is a good method if we are indeed getting correct answers.