To find the unit rate of active minutes per class, you can divide the total active minutes by the number of classes attended.
Justin has a total of 72 active minutes for 3 classes:
\[ \text{Active minutes per class} = \frac{\text{Total active minutes}}{\text{Number of classes}} = \frac{72 \text{ minutes}}{3 \text{ classes}} = 24 \text{ minutes per class} \]
Thus, the unit rate of active minutes per class is 24 minutes.