John Gill Commentary Genesis 5:13

"and Kenan lived after he begat Mahalalel eight hundred and forty years, and begat sons and daughters:" — Genesis 5:13 (ASV)

And Cainan lived, after he begat Mahalaleel, eight hundred
and forty years
The Septuagint has seven hundred and forty, which, added to the years given him before, makes the same sum:

and begat sons and daughters ;
as his progenitors did.