To find the rate at which the candle burns, we need to divide the total height of the candle by the total time it takes to burn down completely.
The initial height of the candle is 20 inches, and it takes 10 hours to burn down completely.
The rate of burn can be calculated as follows:
\[ \text{Rate} = \frac{\text{Total height}}{\text{Total time}} = \frac{20 \text{ inches}}{10 \text{ hours}} = 2 \text{ inches per hour} \]
Thus, the candle burns at a rate of 2 inches per hour.