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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