X is 7th from the top while Y is 16th from the bottom in the class ranking. If they interchange their positions, then X becomes 21st from the top. What will be the position of Y from the bottom?

30th

X is 7^th from the top and Y is 16^th from the bottom.

It means there are 6 students above X and 15 students below Y.

Now, after interchanging their positions,

X becomes 21^st from the top means there are 20 students above X.

But it is clear that there are 6 students above Y, so,

Students in between X and Y = 20 - 6 - 1 (Y) = 14 - 1 = 13.

Position of Y from bottom = 15 + 1 (X) + 13 + 1 (Y) = 30^th.