Question

a man who travels back to a time before his parents were born and kills his gradfather. that would mean one of his parents could not have been born and therefore he himself could not have been born if so , then how could he have killed his grandfather

Answer 1

He was in present day .... he went back and killed his grandfather. Therefore, his wife must be pregnant so he/she must be born. And if one parent would not have been born the the remaining parent could hv married someone else and produced him.

easy as pie

CHEERS!