In a row of boys, Kapil is eighth from the right and Nikunj is twelfth from the left. When Kapil and Nikunj interchange positions, Nikunj becomes twenty first from the left. Which of the following will be Kapil's position from the right ?  

Answer: B. 17th

Since Kapil and Nikunj interchange places, so Nikunj's new position (21st from left) is the same as Kapil's earlier positon (8th from right).
So, number of boys in the row = (20 + 1 + 7) = 28.
Now, Kapiil's new position is the same as Nikunj's earlier position i.e. 12th from left.
.'. Kapil's position from the right = (28 - 11) = 17th.