Two sections of a school took the same test…

Step 1. Section A. $N = 30$. The class marks are $25, 35, 45, 55, 65$ and $u = -2, -1, 0, 1, 2$. $fu = -6, -6, 0, 8, 8$, so $\sum fu = 4$. The mean is $45 + 10 \times \dfrac{4}{30} = 46.33$.

Step 2. Section B. $N = 20$. $fu = -4, -4, 0, 5, 4$, so $\sum fu = 1$. The mean is $45 + 10 \times \dfrac{1}{20} = 45.5$.

Step 3. Together. There are $50$ students. Section A has $\sum fx = 30 \times 46\tfrac{1}{3} = 1390$ and Section B has $\sum fx = 20 \times 45.5 = 910$. The combined mean is $\dfrac{1390 + 910}{50} = \dfrac{2300}{50} = 46$.