For a group of 50 male workers the mean and standard deviation of their daily wages are 
Rs630, Rs 90, respectively. For a group of 40 female workers these are Rs 540 and Rs 60 respectively. The standard deviation of these 90 workers =…………………..
 

Given  $n_1=50,n_2=40,{\overline{x}}_1=630,{\overline{x}}_2=540,{\sigma }_1=90,{\sigma }_2=60$
Combined variance = ${\sigma }^2=\frac{n_1({\sigma }_1^2+d_1^2)+n_2({\sigma }_2^2+d_2^2)}{n_1+n_2}$  where  $d_1=\overline{x}-{\overline{x}}_1,d_2=\overline{x}-{\overline{x}}_2$
Where  $\overline{x}=\frac{n_1{\overline{x}}_1+n_2{\overline{x}}_2}{n_1+n_2}=\frac{50\left(630\right)+40\left(540\right)}{50+40}$   $\overline{x}=\frac{31,500+21,600}{90}=\frac{53,100}{90}=590$

$\begin{array}{l}Now{\sigma }^2=\frac{50(8100+1600)+40(3600+2500)}{90}\\ \frac{50\left(9700\right)+40\left(6100\right)}{90}=\frac{4,85,000+2,44,000}{90}\\ =\frac{72,900}{90}=8100\end{array}$
Now  ${\sigma }^2=8100$
$\sigma =$  combined standard deviation $\sigma =\sqrt{8100}\therefore \sigma =90$