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 72 active minutes and attended the class 3 times:
\[ \text{Unit rate} = \frac{\text{Total active minutes}}{\text{Number of classes}} = \frac{72 \text{ minutes}}{3 \text{ classes}} = 24 \text{ minutes per class} \]
So, the unit rate of active minutes per class is 24 minutes.