Consider a frequency distribution of customers who bought cameras at different price ranges. The table shows that three customers bought cameras that cost from 40 to 59 dollars.

To find out how many customers bought cameras that cost less than 60 dollars, calculate the cumulative frequency for the first two classes. It is given by the sum of all the frequencies of the current and the preceding class intervals. 

Similarly, the remaining cumulative frequencies are estimated using the same method.

Note that the upper and the lower class limits of the adjacent classes differ by the value of one. These gaps are bridged by calculating the midpoint of these class limits, resulting in the class boundaries.

Notice that there is a difference of 20 between each class boundary. Therefore, subtract 20 from the first and add 20 to the last class boundaries to obtain the start and endpoints, respectively.