In a class of 45, Ranjana’s rank is 15^th from first, what is her rank from the last?

31

Total number of students in the class = 45

                  Ranjana’s rank from first = 15^th

                  total number of students=first rank +last rank -1

                                                     45=15+last rank-1

                                            45+1-15=last rank

                                                    31=last rank