Answer:
4.19
Step-by-step explanation:
Find the mean:
[tex]\text{Mean}=\frac{12+19+9}{3}=\frac{40}{3}[/tex]
Find the standard deviation:
[tex]\text{Standard Deviation}=\sqrt{\frac{(12-\frac{40}{3})^2+(19-\frac{40}{3})^2+(9-\frac{40}{3})^2}{3}}=\sqrt{\frac{158}{9}}\approx4.19[/tex]
