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Finding the aprroximated mean of grouped data

Approximate the mean of the grouped data:
The age distribution of students, i, a statistics class.
Age: 20-29 30-39 40-49 50-59 60-69
Frequency: 24 21 21 22 6
What is the mean age of the students based on this given frequency distribution?
(round to the nearest hundredth)

Solution This solution is FREE courtesy of BrainMass!

We take the average of two endpoints in each group ,and then multiply it by the relavtive frequency, like this:

((20+29)/2) * 24/(24+21+21+22+6) + ((30+39)/2) * 21/(24+21+21+22+6)
+((40+49)/2) * 21/(24+21+21+22+6) + ((50+59)/2) * 22/(24+21+21+22+6)
+((60+69)/2) *6/(24+21+21+22+6)
=40.7766

which is the mean average age.