The standard deviation σ is given by:
[tex]\begin{gathered} \sigma=\sqrt{np(1-p)} \\ \text{ Where} \\ p=\text{ the probability of success} \\ n=\text{ number of trials} \end{gathered}[/tex]Substitute n = 420 and p = 0.90 into the formula:
[tex]\sigma=\sqrt{420\cdot0.90(0.10)}\approx6.15[/tex]Hence the correct answer is 6.15
Second Choice
