Ahypothetical element consists of 3 isotopes
with the following masses and natural abundances.
isotopic natural
mass abundance
number %
1 39 78.8
2 40 18.1
3 41 3.1
what is the atomic weight

To calculate the average atomic weight, each exact atomic weight is multiplied by its percent abundance (expressed as a decimal). Then, add the results together and round off to an appropriate number of significant figures

39 x .788 =  30.732
40 x .181 =   5.43
41 x .031 =+ 1.24
                   
                   39   since there are only 2 significant figures given for each mass