The prime numbers in the list are 23 and 29.
The arithmetic mean of these two numbers is $(23+29)/2 = \boxed{26}$.The answer is: 26
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
Find the arithmetic mean of the prime numbers in this list:      21, 23, 25, 27, 29

### Answer:
