The total of the ages of the 11 women is 11·31 = 341. The total of the ages of the 6 men is 6·29 = 174. Then the total of the ages of all 17 people in the office is ...
341 + 174 = 515
and their average age is ...
515/17 ≈ 30.294 ≈ 30
The mean age of the people in the office, to the nearest year, is 30.
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The mean is the total divided by the number of contributors. Then the total is the mean times the number of contributors.