The class width is the difference between the lower limits of any two consecutive classes. For context, the lower limit is the smallest value of a class, and the lower limit is the highest value of the class. All classes should have the same width.
In this case, we can consider the hour intervals as the classes, so if we focus on the 3.0 to 6.4 hours interval and on the 6.5 to 9.9 interval, we can determine the class width:
If we do the same thing with any two consecutive classes, we'll see that the width is also 3.5 (10.0-6.5=3.5, for example).
In conclusion, the class width is 3.5.