A metal rod of unknown length and material increases its length by 5 mm when its temperature is raised by 10°C. What is the rate of change of the rod's length per degree Celsius?

The increase in length, $\Delta L = 5 \text{ mm}$.
The increase in temperature, $\Delta T = 10^\circ \text{C}$.

This value indicates that the rod's length increases by $0.5 \text{ mm}$ for every $1^\circ \text{C}$ increase in temperature.