In a row of boys, if A who is tenth from the left and B who is ninth from the right interchange their positions.. A becomes fifteenth from the left. How may boys are there in the row?

23

According to the question:

A is 10th from the left and B is 9th from the right. After they interchange the position A becomes 15th from left. So, according to the statement we can conclude that total number of boys are 9 + 15 - 1 = 23 (as 1 position is being counted twice). Thus, total number of students are = 23