Given, $n_{1}=25, \bar{x}_{i}=18.2, \sigma_{1}=3.25$

$n_{2}=15, \sum_{i=1}^{15} x_{i}=279$ and $\sum_{i=1}^{15} x_{i}^{2}=5524$

For first set $\Sigma x_{i}=25 \times 18.2=455$

$\therefore$

$\sigma_{1}^{2}=\frac{\Sigma x_{i}^{2}}{25}-(18.2)^{2}$

$\Rightarrow \quad(3.25)^{2}=\frac{\Sigma x_{i}^{2}}{25}-(18.2)^{2} \Rightarrow 10.5625+331.24=\frac{\Sigma x_{i}^{2}}{25}$

$\Rightarrow \quad \Sigma x_{i}^{2}=25 \times(10.5625+331.24)=25 \times 341.8025=8545.0625$

For combined SD of the 40 observations, $n=40$.

Now $\quad \sum_{i=1}^{40} x_{i}^{2}=5524+8545.0625=14069.0625$

and $\quad \sum_{i=1}^{40} x_{i}=455+279=734$

$\therefore \quad SD =\sqrt{\frac{14069.0625}{40}-\left(\frac{734}{40}\right)^{2}}=\sqrt{351.1726-(18.35)^{2}}$

$=\sqrt{351.726-336.7225}=\sqrt{15.0035}=3.87$