Three isotopes of an element have mass numbers m, (m + 1) and (m + 2). If the mean mass number is (m + 0.5) then which of the following ratios may be accepted for m ,(m + 1) , (m + 2) in that order?

mean mass number                         =                                                                =                                                                   =                                                                         =                                                                          =  (m + 0.5)