What is the relation between A.M.U and relative atomic mass?
One atomic mass unit (a.m.u.) is defined as a mass exactly equal to one twelfth the mass of one carbon - 12 atom.
Relative atomic mass:-The relative mass of an atom is its mass compared to the 12C isotope. In other words the standard unit is defined by the carbon 12 atom, which is assigned a value of 12 units. Everything else is measured with respect to this atom and is then described as relative atomic mass.
For example, the mass of a magnesium atom is 2 x the mass of a carbon-12 atom and therefore it has a relative mass of 24.