The following table shows the mean and standard deviation of the lengths of 75 adult puff adders (Bitis arietans), which are found in Africa and on the Arabian peninsula.

a. Find the mean length of the 75 puff adders.

b. The lengths of individual African puff adders are denoted by x_f and the lengths of individual Arabian puff adders by x_b. By first finding Σx²_f ­and Σx²_b, calculate the standard deviation of the lengths of all 75 puff adders.

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Frequency Mean (cm) SD (cm) African 60 102.7 6.8 Arabian 15 78.8 4.2