Since no probability is given, we denote the mean of x as

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