For a group of 50 male workers, the mean and S.D of their daily wages are Rs 630 and Rs. 90 respectively. For a group of 40 female workers, these are Rs 540, Rs 60 respectively. The S.D of these 90 workers is 

$$\begin{gathered} From the given data \\ {n}_1=50 and \overline{x_1}=630 , {\sigma }_1=90 \\ {n}_2=40 and \overline{x_2}=540, {\sigma }_2=60 \\ combined mean \overline{x}=\frac{n_1\overline{x_1}+n_2\overline{x_2}}{n_1+n_2}=590 \\ Now d_1=\overline{x}-\overline{x_1}=-40 and d_2=\overline{x}-\overline{x_2}=-50 \\ combined variance=\frac{n_1({{\sigma }_1}^2+{d_1}^2)+n_2({{\sigma }_2}^2+{d_2}^2)}{n_1+n_2}=8100 \end{gathered}$$