Enriched weapons-grade uranium consists of 80% uranium-235 (235.044 amu) and 20% uranium-238 (238.051 amu). what is the average atomic mass of weapons-grade uranium, assuming the percentages are exact?

Isotopes are atoms of the same element that have the same number of protons with different numbers of neutrons, hence they have different masses.
the natural abundance of these elements vary, we are given percentages in which these isotopes exist.
average atomic mass is the weighted average atomic mass of an atom relative to 1/12th the mass of a C-12 atom. the average atomic mass can be calculated by taking the sum of the masses of isotopes multiplied by the respective abundance percentage 
average atomic mass = (235.044 amu x 80%) + (238.051 amu x 20%)
                                   = 188.04 + 47.61
                                   = 235.65 amu
the average atomic mass is 235.65 amu 

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