• Sounds like a college homework assignment?
  • If the primes are roughly the same size, they should be around 200 digits each (maybe only 199 digits). Or it could be a combination such as a 100 digit number and a 300 digit number. The number of digits is proportional to the log of the number, hence they are roughly additive. In any case, there are probably many thousands of pairs of primes whose product is 400 digits. (And 'thousands' is probably a gross underestimate!) More information is needed in order to answer this question. So I think you are mistaken to speak of 'the' two primes as though there is a unique pair whose product is 400 digits. Composite numbers that are the product of two large primes form the basis of public-key cryptography. They are intensively studied and cataloged for this reason.

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