Did you get it right?
The answer is:
40Lb: 40 amu x 0.410 = 16.4 42Lb: 42 amu x 0.483 = 20.286 43Lb: 43 amu x 0.107 = 4.601
16.4 + 20.286 + 4.601 = 41.287 amu
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