A short project is defined by activities with three time estimates $($in days$)$ as shown in the table.

Activity

Optimistic

Most Likely

Pessimistic

Predecessor

A

$4$

$6$

$8$

$$

B

$5$

$9$

$13$

$$

C

$10$

$12$

$15$

A$,$ B

D

$9$

$12$

$16$

B

E

$6$

$9$

$15$

D

F

$10$

$13$

$18$

C

What is the standard deviation of the project's completion time?