Three arithmetic means are inserted between 5 and 29. The second of those means is:

Inserting three means makes an arithmetic progression of five terms running from 5 to 29, so 29 = 5 + 4d and d = 6. The progression is 5, 11, 17, 23, 29 and the second mean is 17. (11 and 23 are the first and third means. Dividing the gap by 3 — the number of means — instead of by 4 gives d = 8 and the sequence 5, 13, 21, 29, whose second entry 21 is the classic wrong answer: three means need four equal steps, not three.)