Mr. Li devises a new slingshot that he wants to market to students for pulling graduation pranks. During his testing, he finds that it has a mean firing distance of 50.6 metres with a standard deviation of 7.8 metres. He does 50 firing tests. Find a 90% confidence interval for the actual mean firing distance of the slingshot. What distance should Mr. Li use to truthfully advertise for his slingshot?