A is 8th from the top while B is 15th from the bottom in the class ranking. If they interchange their positions, then A becomes 22nd from the top. What will be the position of Y from the bottom?

29

A's position from top = 8th

B's position from bottom = 15th

when they interchange then A's position becomes 22nd from top.

Ist situation after interchange

A's position is 15th from bottom 

B's position 8th from top

and in the question A's position 22nd from top

Therefore:

first of all we calculate

Total number of students = top + bottom - 1

                                        =  22 + 15 - 1

                                        = 36

IInd situation:

For Calucating position of  b's from bottom

Total number of students = top + bottom - 1
                                    36 =  8 + bottom - 1

                              bottom =  36  - 8 + 1

                                          =  29 th