For P_X Use

Percentile Point (X) = X_L + (i/F_i)(cum F_p – cum F_L)

For PR_X Use

                                cum F_L + ( F_i / i )(X – X_L)

       Percentile Rank (X) = ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ ×i100

                                                    N