1. The problem is to find the mean (average) mark for group b. 2. The mean is calculated using the formula: $$\text{Mean} = \frac{\text{Sum of all marks}}{\text{Number of marks}}$$ 3. To find the mean mark for b, add all the marks of group b and then divide by the number of marks in group b. 4. Suppose the marks for b are $b_1, b_2, b_3, \ldots, b_n$, then: $$\text{Mean for b} = \frac{b_1 + b_2 + b_3 + \cdots + b_n}{n}$$ 5. Calculate the sum of the marks and divide by $n$ to get the mean. 6. This gives the average mark for group b, which represents the central tendency of the marks in that group.