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The total wingspan for data set A is 6 x 13 = 78 cm so the missing measurement is 78 - 67 = 11 cm.

The total wingspan for data set B is 7 x 14 = 98 cm so the missing measurement is 98 - 84 = 14 cm.

The overall mean is 13.35 cm so the total of all 20 measurements is 20 x 13.35 = 267 cm and the total wingspan for data set C is 267 - (78 + 98) = 91 cm.

The missing measurement for data set C is 91 - 76 = 15 cm and the mean of data set C is 91/7 = 13 cm.