Answer: In mathematics, the additive inverse of a number is the number that, when added to the original number, results in zero.
Step-by-step explanation: The additive inverse of 21.9 can be found by simply negating the value, which means changing the sign from positive to negative, or vice versa. Therefore, the additive inverse of 21.9 is -21.9, because 21.9 + (-21.9) = 0.