Jenny's phone number is 867-5309, according to the song. Here are some problems about the number 8675309.
1. 8675309 is prime. How many prime phone numbers are there if phone numbers are 7 digits and cannot start with 0?
2. How many prime phone numbers are there if phone numbers are 7 digits and can start with 0?
3. 8675309 is the 582161st prime, and 582161 is prime, too. Therefore, 8675309 is a superprime. How many superprimes are ≤8675309?
4. 8675311 is prime. Therefore, 8675309 and 8675311 are twin primes. How many twin prime pairs (n, n+2) are there such that n≤8675309?