We know n is 24, and . Therefore,
Your problem gives
That can be rewritten as .
<------ from above
I'll take a stab at part (ii) as well.
From above, for your second equation, substitute our value of 288 for a, we get
Expanding and simplifying, we get:
<------ remember that from part i
Now, we know that Variance, denoted as
Plugging in our value for and simplifying, we get:
<---- we figured out earlier and
Now calculate standard deviation, denoted as