If the sum of the deviations of 50 observations

$\sum_{i=1}^{50}\left(x_{i}-30\right)=50$

$\Sigma x_{i}=50 \times 30=50$

$\Sigma x_{i}=50+50+30$

Mean $=\bar{x}=\frac{\sum x_{i}}{n}=\frac{50 \times 30+50}{50}=30+1=31$