A data set has the following frequency distribution: Class interval 10-20 has a frequency of 5, class interval 20-30 has a frequency of 8, class interval 30-40 has a frequency of 3, and class interval 40-50 has a frequency of 4. Find the class boundaries of the frequency table.

Step 1 ：First, we find the class width by subtracting the lower class limit of the second class from the lower class limit of the first class. Using the given frequency distribution, the class width is $20-10=10$.

Step 2 ：The lower class boundary of the first class is the lower class limit of the first class subtract half of the class width. Therefore, the lower class boundary of the first class is $10-0.5\times 10=5$.

Step 3 ：The upper class boundary of the first class is the upper class limit of the first class plus half of the class width. Therefore, the upper class boundary of the first class is $20+0.5\times 10=25$.

Step 4 ：Using the same method, we can calculate the class boundaries for the other classes.