To find the average number of students who would benefit from tutoring, you can calculate the midpoint between the lower and upper margins given.
The lower margin is 13 and the upper margin is 21.
To find the average:
\[ \text{Average} = \frac{\text{Lower Margin} + \text{Upper Margin}}{2} \]
\[ \text{Average} = \frac{13 + 21}{2} = \frac{34}{2} = 17 \]
Thus, the average number of students in the class who said they would benefit from tutoring is 17.