A first order reaction has a rate constant of $2.303\times {10}^{-3}{s}^{-1}.$ The time required for 40g of this reactant  to reduce to 10gm will be  $[\log 2=0.3010]$

$\mathrm{k}=2.303\times {10}^{-3}{\mathrm{J}}^{-1}, \mathrm{a}=40\mathrm{g},(\mathrm{a}-\mathrm{x})=10\mathrm{g}$

$\begin{array}{l}\mathrm{For}{1}^{\mathrm{st}}\mathrm{order}\mathrm{reaction}, \mathrm{t}=\frac{2.303}{\mathrm{k}}{\log }_{10}\frac{\mathrm{a}}{(\mathrm{a}-\mathrm{x})}\\ =\frac{2.303}{2.303\times {10}^{-3}}{\log }_{10}\left(\frac{40}{10}\right)\\ \mathrm{t}=602\sec \end{array}$