**Answer:**

And the 95% confidence would be :

**Step-by-step explanation:**

We have the following info given from the problem

sample mean for this year

sample deviation for this year

random sample selected for this year

sample mean for last year

sample deviation for last year

random sample selected for last year

And we want to construct a 95% confidence interval estimate of the difference μ1−μ2, where μ1 is the mean of this year's sales and μ2 is the mean of last year's sales

For this case the formula that we need to use is:

The degrees of freedom are given by:

The confidence level is 0.95 and the significance would be and so then the critical value for this case is :

The margin of error would be:

And the confidence interval would be given by:

And the 95% confidence would be :