Answer
Class width = 50
12 classes are represented in the graph.
Work Step by Step
The x-coordinate of each point represents the upper limit of the corresponding class. But, the first does not represent the upper limit of any class. It is plotted as the "starting point".
In the graph there are 13 points. So, there are $13-1=12$ classes.
The class width is the difference between any two consecutive upper limits: $300-250=350-300=400-350=50$
