The mean of 100 observations is 50. If one of the observations, 50 is replaced by 150, then the resulting mean is

$$\begin{gathered} \mathrm{sum}=\frac{\mathrm{Sum} \mathrm{of} \mathrm{observations}}{\mathrm{No}. \mathrm{of} \mathrm{observations}} \\ 50=\frac{\mathrm{sum}}{100} \end{gathered}$$
sum  = 5000
if $50 \mathrm{replaced} \mathrm{by} 150$
$$\begin{gathered} \mathrm{resulting} \mathrm{sum} =5000-\left(50\right)+150 = 5100 \\ \mathrm{resulting} \mathrm{mean}=\frac{5100}{100}=51 \end{gathered}$$