The mean of first ten prime numbers is:

The arithmetic mean = Sum of all terms/ Number of terms

According to the question,

First 10 prime numbers = 2, 3, 5, 7, 11, 13, 17, 19, 23, 29

The arithmetic mean = (2 + 3 + 5 + 7 + 11 + 13 + 17 + 19 + 23 + 29)/10

⇒ 129/10 = 12.9