Final answer:
Without specific sales data or results from a statistical test, it's impossible to determine if there's a significant difference between the mean sales of two newspaper vendors.
Step-by-step explanation:
To determine whether there is a significant difference between the mean sales of two newspaper vendors, we typically use statistical analysis, such as a t-test for comparing two means. However, we are not provided with specific data or results from such a test in this scenario. We have general information about the industry, like the nationwide availability of certain newspapers and the trend of declining sales and advertising revenue, which suggests that sales may differ among vendors. In addition, we know that variance exists in the cost of newspapers from city to city. Yet, without precise sales data from the two vendors in question or the results of a statistical test, we cannot conclusively answer whether there is a significant difference between their mean sales.