Dang! So close to your solution, but i forgot about the use of modulo and tried real division and test with is_integer. Didn't work out well, since the division produced errors if integers became too large...

Nothing strange. We have random numbers in tests, and there is un-efficient code for prime-checking.

Duplicate.

When I tried my solution, system told me that "Process was terminated. It took longer than 6000ms to complete
". I doubt that my solution is inefficient.
Then I copy your code, run again, then I got the the time-out question again; I can't submit it.
Are there some questions with the Computer or speed of the Internet?
So strange.

Typo in Details

The best solution that actually helps me to learn code, thanks!