Determine whether the argument is valid for each situation. Explain your thinking. a) An advertising company has a mean monthly sales record of $16 235. Therefore, half the team members sold more than $16 235. b) A survey shows that 78% of all salaries are below the mean. Therefore, there must be a mistake. c) The mean mark of one class is 71, while the mean mark of another class is 76. Therefore, the mean of the two classes is 73.5. d) My median monthly expenses total $850. Therefore, my total expenses for the year must be $10 200.