Answer:
The third number can be any number less than 30.
Explanation:
If the arithmetic mean of three numbers is less than 20, then their sum must be less than 20 * 3, or 60. We know that two of the numbers are 5 and 25, which add up to 30. In order for the arithmetic mean of the three numbers to be less than 20, the unknown number must be less than (60 - 30), or in order words, less than 30. So the third number can be any number less than 30.