Answer :
we must use the formula
[tex]\sqrt[]{\frac{\sum^N_i(x_i-X)^2}{N}}[/tex]where N is the total of numbers xi each value and X the average
CALCULATIONS
X
[tex]X=\frac{8+9+12+15+18+21+22}{7}=15[/tex]Sum of (xi-X)
[tex]\begin{gathered} x1=8-15=-7 \\ x2=9-15=-6 \\ x3=12-15=-3 \\ x4=15-15=0 \\ x5=18-15=3 \\ x6=21-15=6 \\ x7=22-15=7 \end{gathered}[/tex][tex]x1+x2+x3+x4+x5+x6+x7=0[/tex]STANDARD DEVIATION
[tex]\sqrt[]{\frac{0}{7}}=0[/tex]THE STANDARD DEVIATION IS 0