Is there a function in R that calculate the mean and median for a grouped
data?
For example, a survey shows the oil price outlook in the future. How can I
calculate the mean/median?
(Of course, I understand that the groups "below 80" and "above 110" must be
defined more specifically)

  below 80 4  80-90 5  90-100 8  100-110 7  above 110 3

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