Given the total frequency is 20, the equation for the sum of frequencies is 13 + a + b = 20, which simplifies to a + b = 7. The median class for a total frequency of 20 is the class containing the 10th value, which falls in the 20 to 30 interval. Using the grouped median formula, Median = L + ((N/2 - CF) / f) * h, we substitute the known values: 22.5 = 20 + ((10 - (4 + a)) / a) * 10. Simplifying this gives 2.5 = ((6 - a) / a) * 10, which results in 0.25 = (6 - a) / a, meaning a = 24 - 4a, so 5a = 24; however, using the correct cumulative frequency preceding the median class of 8 + a gives 22.5 = 20 + ((10 - (8 + a)) / a) * 10, which simplifies to 2.5a = 20 - 2a, resulting in 4.5a = 22.5 and a = 5.