Where does your "50" come from? Don't forget that a number of inhabitants is an integer.

1. Always mark your solutions as spoiler (even if they are faulty, they have a lot of spoilers)
2. You are breaking the loop before j = False gets executed. So, j is never False. And your function fails.

We were answering simultaneously CrazyMerlin. Sorry for that!

You're generating two lists of primes. Why? This will cost you a lot of time. When you have a problem of prime numbers you generally may have two situations:

• you need a fast prime generator and in some cases to check if the number is prime, but will be used only once per run

• you need a fast primality test checker like Miller - Rabin or others, this kind of test cost a lot of time. It should be used carefully and conveniently.

Try to optimize your code. Use spoiler to present your solutions!!

Calculating all primes below n will take too long no matter how much you accelerate it.

Maximum of the primes smaller than n is the one closest to it.
You don't need to check numbers lower than that.