11. The general manager of a chain of fast food chicken restaurants wants to determine how effective their promotional campaigns are. In these campaigns, "20% off' coupons are widely distributed. These coupons are only valid for one week. To examine their effectiveness, the executive records the daily gross sales (in $1,000s) in one restaurant during the campaign and during the week after the campaign ends. The data are shown below: Day Sales During Campaign Sales After Campaign Sunday 18.1 16.6 Monday 10.0 8.8 Tuesday 9.1 8.6 Wednesday 8.4 8.3 Thursday 10.8 10.1 Friday 13.1 12.3 Saturday 20.8 18.9 Assume that the differences between pairs of sales are normally distributed. (a) Construct a 95% confidence interval for the difference between the mean sales during campaign and the mean sales after campaign. (b) Can you be 95% confident that sales increase during the campaign