Mathematics · Data Interpretation

In a survey of students changing subjects between two rounds, let variables 'a' and 'b' be defined as follows: a = The number of students who selected subject B in the first round but chose subject A in the second round. b = The number of students who selected subject B in the first round but chose subject C in the second round. If the total number of students who selected subject B in the first round is 30, and all of them switched to either A or C in the second round such that the ratio of a to b is 7:8, what is the minimum value of |a - b|?