Final answer:
The change in temperature from -X°C to -Y°C is calculated by the expression Y - X, which represents the correct change in degrees Celsius.
Step-by-step explanation:
The question asks about calculating the change in temperature from one day to the next. To find the change in temperature, we subtract the initial temperature from the final temperature. In this case, the initial temperature was -X°C and the final temperature was -Y°C. The change in temperature (ΔT) is therefore calculated by (-Y) - (-X), which simplifies to -Y + X.
Therefore, the correct mathematical expression for the change in temperature is Y - X.