What is the average of the prime numbers lying between 20 and 40?

The primes between 20 and 40 are 23, 29, 31 and 37 - four of them, adding to 120, so the average is 120/4 = 30. 29 comes from also counting 25 as a prime, which gives 145/5 = 29; but $25=5\times 5$. 24 divides the correct total of 120 by 5 instead of by 4, again from miscounting how many primes there are. 34 is the average of just 31 and 37, the two largest, leaving out the smaller half of the list.