Answer:
The 99% confidence interval for the unknown mean sodium level is between 100.42 milligrams of sodium per slice and 105.58 milligrams of sodium per slice.
Step-by-step explanation:
We have that to find our
level, that is the subtraction of 1 by the confidence interval divided by 2. So:

Now, we have to find z in the Ztable as such z has a pvalue of
.
That is z with a pvalue of
, so Z = 2.58.
Now, find the margin of error M as such

In which
is the standard deviation of the population and n is the size of the sample.

The lower end of the interval is the sample mean subtracted by M. So it is 103 - 2.58 = 100.42 milligrams of sodium per slice.
The upper end of the interval is the sample mean added to M. So it is 103 + 2.58 = 105.58 milligrams of sodium per slice.
The 99% confidence interval for the unknown mean sodium level is between 100.42 milligrams of sodium per slice and 105.58 milligrams of sodium per slice.