Answer:
We need at least 243 stores.
Step-by-step explanation:
In a sample with a number n of people surveyed with a probability of a success of
, and a confidence level of
, we have the following confidence interval of proportions.

In which
z is the zscore that has a pvalue of
.
The margin of error of the interval is:

For this problem, we have that:

95% confidence level
So
, z is the value of Z that has a pvalue of
, so
.
Determine the number of stores that must be sampled in order to estimate the true proportion to within 0.04 with 95% confidence using the large-sample method.
We need at least n stores.
n is found when M = 0.04. So






Rounding up
We need at least 243 stores.
Addition will be the last operation
To get from 40% to 100%, divide by 2 and multiply by 5.
I.e. 40/2 = 20
20 x 5 = 100%
Do the exact same for 102
102/2 = 51
51 x 5 = 255 students