It's only the primes above 50 that are immediately eliminated by the fact that Peter does not know the answer right away. The primes below that can be present

It's only the primes above 50 that are immediately eliminated by the fact that Peter does not know the answer right away. The primes below that can be present in products that have multiple possible answers: eg. we can't eliminate 47 right off the bat because Peter could have been told a product like 3290.