To find the joint relative frequency of students who use a mobile device in an online class, you can use the formula:
\[ \text{Joint Relative Frequency} = \frac{\text{Number of students using mobile device in online class}}{\text{Total number of students}} \]
From the two-way frequency table, the number of students who use a mobile device in an online class is 64, and the total number of students is 200.
Now, we can calculate the joint relative frequency:
\[ \text{Joint Relative Frequency} = \frac{64}{200} = 0.32 \]
To express this as a percentage:
\[ 0.32 \times 100 = 32% \]
Therefore, the joint relative frequency of students who use a mobile device in an online class is 32%.