Initially, A is 10th from the left and B is 9th from the right.
After A and B interchange their positions, A becomes 15th from the left.
Since A moves to B's original position, B's original position must be 15th from the left.
Therefore, B is both:
- 15th from the left
- 9th from the right
For any person in a row:
Total number = (Position from left) + (Position from right) - 1
Total number of boys =15+9−1
Total number of boys =23
Therefore, there are 23 boys in the row.
The answer is Option 1: 23.