Explanation:

- Total students in Class A and B combined are 72.

- If number of students in A is \( x \), then in B it is \( y \). Given \( x + y = 72 \) and \( x = 1.4y \).

- From \( x = 1.4y \), calculate \( y = 30 \) and \( x = 42 \).

- The average marks in B are 1.5 times those in A.

- Let average in A be \( m \), then in B it's \( 1.5m \).

- Aggregate average for both classes is 58: \((42m + 30 \times 1.5m) / 72 = 58\).

- Solving gives \( m = 48 \), so B's average is \( 1.5 \times 48 = 72 \).

- Option 1: 72 - This matches our result.

- Option 2: 58 - This is not B's average.

- Option 3: 50 - B's average is not 50.

- Option 4: 48 - This is A's average.