Genesis 50:3 - Matthew Poole's English Annotations on the Holy Bible

For him, i.e. for his embalming; that so the drugs or spices which were applied might more effectually reach to all the parts of the dead body, and keep it from corruption. And the effect of their diligence and so long continuance in this work was, that bodies have been preserved uncorrupt for some thousand of years. Threescore and ten days, i.e. thirty days, (according to the custom of the Hebrews, Numbers 20:29 Deuteronomy 34:8, to which doubtless the Egyptians in this case did accommodate themselves,) besides the forty days spent in embalming him, which also was a time of mourning. And thus I suppose the Egyptians reckoned those seventy-two days which Diodorus Siculus saith they spent in mourning for their deceased kings.