Your answer, "800,000", is not correct.
The correct answer to
According to Joab, how many men of fighting age lived in Israel?
is
1,100,000
as shown by
1 Chronicles 21:5
which is
- KJV
- And Joab gave the sum of the number of the people unto David. And all they of Israel were a thousand thousand and an hundred thousand men that drew sword: and Judah was four hundred threescore and ten thousand men that drew sword.
- NIV
- Joab reported the number of the fighting men to David: In all Israel there were one million one hundred thousand men who could handle a sword, including four hundred and seventy thousand in Judah.