1. **State the problem:** Add the numbers $5.5$ and $-2.5$. 2. **Recall the rule for adding positive and negative numbers:** When adding a positive number and a negative number, subtract the smaller absolute value from the larger absolute value and take the sign of the number with the larger absolute value. 3. **Apply the rule:** $$5.5 + (-2.5) = 5.5 - 2.5$$ 4. **Perform the subtraction:** $$5.5 - 2.5 = 3.0$$ 5. **Final answer:** $$5.5 + (-2.5) = 3.0$$ So, the sum of $5.5$ and $-2.5$ is $3.0$.