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 the week and he attended the class 3 times.
The calculation is as follows:
\[ \text{Unit rate} = \frac{\text{Total active minutes}}{\text{Number of classes}} = \frac{72}{3} = 24 \]
Therefore, the unit rate of active minutes per class is 24 active minutes per class.