Genesis 5:13
“And Cainan lived after he begat Mahalaleel eight hundred and forty years, and begat sons and daughters:”
KJV
Read in Genesis 5Meaning
Kenan lives another 840 years after Mahalalel and has additional sons and daughters beyond the named genealogical line.
Read carefully
The genealogy's selectivity should not be mistaken for a complete demographic register of the period it describes.
Reflection
The text repeatedly signals that the story's named line exists inside a much broader network of families and descendants.