15.6
Step-by-step explanation:
First, multiply the midpoint of each class by its frequency, as follows:
      Class    midpoint  frequency  midpoint*frequency Â
      0-9     4.5       24         4.5*24 = 108
      10-19    14.5       20         14.5*20 = 290
      20-29   24.5      32         24.5*32 = 784
Total                    76        1182
The mean is computed as the division between the addition of the "midpoint*frequency" column by the addition of "frequency" column.
mean = 1182/76 ≈ 15.6
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