The average score in Mathematics of 90 students of sections A and B of class IX was 63. The number of students in A were 10 more than those in B. The average score of students in A was 30% more than that of students in B. The average score of students in B is:

Let students in section B = x, then section A = x + 10. Total: x + x + 10 = 90, so 2x = 80, x = 40. Section B has 40 students, section A has 50 students. Let average of B = a, then average of A = 1.3a (30% more). Weighted average: (50 × 1.3a + 40 × a) / 90 = 63. So (65a + 40a) / 90 = 63, 105a = 5670, a = 54. Section B's average is 54. This is a standard weighted average problem.