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.