Durations of minutes of a sample of long-distance phone calls within the continental United States reported by one long-distance carrier along with their relative frequency is given in table as follows,

The table of time in terms of class intervals is given as,

The width of classes is given as difference of lower and upper limit of each class.

On inspection, it can be seen that difference between any of the given class limits is 5.

For instance,

U.L−L.L=20−15=5U.L−L.L=20−15=5

Hence, the class width is 5 for each class.