1. **State the problem:** We need to find how many degrees Celsius the temperature of mercury must drop to go from room temperature (22°C) to the freezing point of dry ice (–78.5°C). 2. **Formula used:** The temperature change $\Delta T$ is calculated by subtracting the final temperature from the initial temperature: $$\Delta T = T_{initial} - T_{final}$$ 3. **Apply the values:** $$\Delta T = 22 - (-78.5)$$ 4. **Simplify the expression:** $$\Delta T = 22 + 78.5 = 100.5$$ 5. **Interpretation:** The mercury needs to cool down by 100.5 degrees Celsius to reach the freezing point of dry ice. **Final answer:** The temperature of the mercury must drop by **100.5°C**.