If both the mean and the standard deviation

$\operatorname{Mean}(\mu)=\frac{\sum \mathrm{x}_{\mathrm{i}}}{50}=16$

standard deviation $(\sigma)=\sqrt{\frac{\sum \mathrm{x}_{\mathrm{i}}^{2}}{50}-(\mu)^{2}}=16$

$\Rightarrow(256) \times 2=\frac{\sum x_{i}^{2}}{50}$

$\Rightarrow$ New mean

$=\frac{\sum\left(x_{i}-4\right)^{2}}{50}=\frac{\sum x_{i}^{2}+16 \times 50-8 \sum x_{i}}{50}$

$=(256) \times 2+16-8 \times 16=400$