This puzzle is a little technical. It involves prime numbers. A number is a prime if its only factors are itself and one. So 6 is not a prime, as it has factors of 2 and 3. 7 is a prime. There is a little more about primes here.
This puzzle looks at the sequence 31, 331, 3331, etc and sees if each number is a prime.
© Jo Edkins 2012 - Return to Puzzles index