This answer is incorrect. The mean date of publication is figured by adding all the dates together and dividing by the number of dates.
1980+1985+1990+1995+2000=9950
9950 / 5 = 1990
The median is figured by determining what date falls in the middle of the range. Two dates are earlier than 1990, and two dates are later than 1990. Therefore, 1990 is the median.