To find the unit rate of active minutes per class, you need to divide the total active minutes by the number of classes attended.
Justin tracked a total of 72 active minutes in the week and attended the class 3 times.
So, the calculation is:
\[ \text{Active minutes per class} = \frac{\text{Total active minutes}}{\text{Number of classes}} = \frac{72 \text{ minutes}}{3 \text{ classes}} = 24 \text{ minutes/class} \]
The unit rate of active minutes per class is 24 minutes.