A frequency table gives the number of weather stations in each temperature class in degrees Celsius as follows: 0 to 10 has 1 station, 10 to 20 has 2, 20 to 30 has 3, 30 to 40 has 4, 40 to 50 has 5, 50 to 60 has 4, 60 to 70 has 3, 70 to 80 has 2 and 80 to 90 has 1, giving 25 stations in all. Which one of the following is the median of the above data?