*On the Réamur scale, which is no longer used, water freezes at $0^{\circ} \mathrm{R}$ and boils at $80^{\circ} \mathrm{R}$. (a) Derive an equation that relates this to the Celsius scale. (b) Derive an equation that relates this to the Fahrenheit scale. (c) Mercury is a liquid metal at room temperature. It boils at $356.6^{\circ} \mathrm{C}$ $\left(673.9^{\circ} \mathrm{F}\right)$. What is the boiling point of mercury on the Réamur scale?