2/5, 0.404040 and 0.286 are all rational as they all can be expressed in fraction form (0.4040 can be expressed as 40/99 and 0.286 as 286/1000 (can simplify but I didn't)) sqrt7 is the only irrational number.
The opposite of a number is its negative opposite. It's the additive inverses of a number. The additive inverse of 4 is -4. It will always equal zero when added together.
