Lithium, sodium and potassium have atomic masses 6.9, 23.0 and 39.0. The average of the first and last is:

1. Add 6.9 and 39.0 to get 45.9. 2. Half of 45.9 is 22.95. 3. So the average is 22.95. _Source: syllabus Science Class X, Chapter 5, Periodic Classification of Elements._