A. The frequency is indicated in the vertical axis.
B. The horizontal axis indicates the classes, which in this case are the test scores intervals.
C. The horizontal axis has a class width of 10.
D. The frequency for the class 60-69 is 3.
E. The frequency for the class 70-79 is 9.
F. The frequency for the class 80-89 is 12.
G. The frequency for the class 90-99 is 7.
H. We can verify that 31 students took the test because we should have 31 data points (test scores) divided into the different classes.
We should obtain 31 when adding the frequency of all the classes:
I. The new scores are: 78, 81, 82, 85 and 88.
So the class 70-79 increases its frequency by one, while the class 80-89 increases its frequency by 4.
We can modify the histogram and obtain: