The mean of the data 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 is

$$\begin{gathered} \mathrm{Mean} =\frac{\mathrm{sum} \mathrm{of} \mathrm{observations}}{\mathrm{numberj} \mathrm{of} \mathrm{observations}} \\ \mathrm{Mean} \mathrm{of} \mathrm{the} \mathrm{data} 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 \mathrm{is} \\ =\frac{16+17+18+19+20+21+22+23+24+25+26}{11} \\ =\frac{231}{11} \\ =21 \end{gathered}$$