Question

How do you estimate 403,446 +396,755

Answer 1

You round both numbers to 1 significant figure. So you do 400,000 + 400,000 = 800,000

Answer 2

403446 is around 400000 so you round that up in your head.  396755 is close to 400000 so you round that up in your head to.  Then you add them together to get 800000, which you can do in your head.