Final answer:
The opposite of 5.5 as a decimal is -5.5. This is known as the additive inverse, which is found by changing the sign of the original number.
Step-by-step explanation:
The question appears to be asking about the mathematical concept of an 'opposite' or 'additive inverse' for the decimal 5.5.
In mathematics, the opposite of a number is what you would add to that number to get zero.
For the number 5.5, its opposite is -5.5.
This principle can be applied to any real number: to find its opposite, simply change the sign in front of the number.