Concept:Calculating the true class size for continuous data grouped into discrete class intervals.Explanation:When we have class intervals like 10-19, 20-29, 30-39, etc., the upper limit of one class is immediately followed by the lower limit of the next class. To make these classes continuous and find the actual size, we adjust the limits.1. Find the difference between the upper limit of one class and the lower limit of the next class. For example, $20 - 19 = 1$.2. Divide this difference by 2 to get the adjustment factor: $\frac{1}{2} = 0.5$.3. Subtract this factor from the lower limit of each class and add it to the upper limit of each class to get the true class limits. - For the first class (10-19): True limits are $10 - 0.5 = 9.5$ and $19 + 0.5 = 19.5$. - For the second class (20-29): True limits are $20 - 0.5 = 19.5$ and $29 + 0.5 = 29.5$. - For the third class (30-39): True limits are $30 - 0.5 = 29.5$ and $39 + 0.5 = 39.5$.4. The size of the class is the difference between the true upper limit and the true lower limit. - Using the third class: Size = $39.5 - 29.5 = 10$. - Alternatively, using the second class: Size = $29.5 - 19.5 = 10$.The size of each class interval is 10.Answer:10