If in a data set, all numbers appear just once, what is the mode?

Well there is no mode or all the #'s are the mode. See if that helps.

In a data set where all numbers appear just once, there is no mode. The mode is the value that appears most frequently in a data set. If each number appears only once, there is no number that appears more frequently than the others, hence there is no mode. To determine the mode in a data set, you need to identify the number(s) that appear(s) most frequently.