Mean of 100 items is 49. It was discovered that three items which should have been 60, 70, 80 were wrongly read as 40

Mean of 100 items is 49

$$\begin{gathered} \frac{{\mathrm{x}}_1+{\mathrm{x}}_2+......+{\mathrm{x}}_{100}}{100}=49 \\ {\mathrm{x}}_1+{\mathrm{x}}_2+........+{\mathrm{x}}_{100}=4900 \end{gathered}$$

By wrongly three observations noted as 40 instead of 60, 70, 80 

$$\begin{gathered} 60+70+80-40-40+{\mathrm{x}}_1+{\mathrm{x}}_2+.......+{\mathrm{x}}_{100} \\ =90+4900=4990 \\ \mathrm{New} \mathrm{Mean} =\frac{4990}{100}=499 \end{gathered}$$