The owner of a golf course wants to determine if his golf course is more difficult than the one his friend owns. He has 10 golfers play a round of 18 holes on his golf course and records their scores. Later that week, he has the same 10 golfers play a round of golf on his friend's course and records their scores again. The average difference in the scores (treated as the scores on his course - the scores on his friend's course) is 8.574 and the standard deviation of the differences is 15.6694. Calculate a 90% confidence interval to estimate the average difference in scores between the two courses.