Answer here !

Relative atomic mass by best Chemistry teacher in Patna , IITian Rishi Sir :

            One of the most important concept come out from Dalton’s atomic theory was that of relative atomic mass or relative atomic weight. This is done by expressing mass of one atom with respect to a fixed standard. Dalton used hydrogen as the standard (H = 1). Later on oxygen (O = 16) replaced hydrogen as the reference. Therefore relative atomic mass is given as

            On hydrogen scale : Relative atomic mass (R.A.M) =  mass of 1 atom with respect to mass of 1 atom of H atom

            On oxygen scale : Relative atomic mass (R.A.M) = mass of 1 atom with respect to 1/16th of mass of 1 atom of O atom

The present standard unit which was adopted internationally in 1961, is based on the mass of one carbon-12 atom.

                        Relative atomic mass (R.A.M) = mass of 1 atom with respect to 1/12th of mass of 1 atom of C-12 atom

            Atomic mass unit (or amu) : 

D2:      The atomic mass unit (amu) is equal to  mass of one atom of carbon-12 isotope.

                            \         1 amu =  × mass of one C-12 atom

                                    » mass of one nucleon in C-12 atom.

                                    = 1.66 × 10^–24 g or 1.66 × 10^–27 kg