20 is not prime, because it has more than two factors: 1, 2, 4, 5, 10, and 20. 21 is not prime, because it has more than two factors: 1, 3, 7, and 21. 22 is not prime, because it has more than two factors: 1, 2, 11, and 22. 23 is prime, because it has exactly two factors: 1 and 23. 24 is not prime, because it has more than two factors: 1, 2, 3, 4, 6, 8, 12, and 24. 25 is not prime, because it has more than two factors: 1, 5, and 25. 26 is not prime, because it has more than two factors: 1, 2, 13, and 26. 27 is not prime, because it has more than two factors: 1, 3, 9, and 27. 28 is not prime, because it has more than two factors: 1, 2, 4, 7, 14, and 28. 29 is prime, because it has exactly two factors: 1 and 29. 30 is not prime, because it has more than two factors: 1, 2, 3, 5, 6, 10, 15, and 30. So the only prime numbers between 20 and 30 are 23 and 29. Therefore, there are two prime numbers between 20 and 30. 
How many prime numbers are between 20 and 30?