Answer:
She needs a sample size of 25.
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 = 0.995.
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 population SD is 2 grams.
This means that 
What is the minimum sample size she needs to create a confidence interval that has a width of 0.4 grams?
She needs a sample size of n.
n is found when M = 0.4. So





Rounding up:
She needs a sample size of 25.