You have to find how many primes are in range from 2 to n that have one property: If you cut last digit, their rest digits will still make prime number.
Example: 2339 -> 233 -> 23 -> 2 all are primes.
PrimaryPrime.find(24) -> 5 // 2,3,5,7,23
Primes from 2 to 24: [2,3,5,7,11,13,17,19,23] The first 4 primes are of course Primary Primes. The 4 primes, that's stars with 1 are NO Primary Primes, because 1 (first digit) is no prime. And 23 is a Primary Prime, because 2 (first digit) is a prime.