To find the unit rate of active minutes per class, you divide the total active minutes by the number of classes attended.
Justin's total active minutes for the week is 72 minutes, and he 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.