15. explain how you can make a prediction for the atomic mass from only the atomic mass of each isotope and the ratios of isotopes that are present.

The average atomic mass can be calculated from the exact atomic mass of each isotope by multiplying the mass each isotope by its relative abundance and then finding the sum of the result.

for example carbon has two major isotopes ;carbon 12 ,atomic mass  with relative abundance of 98.9% and carbon  13 , atomic mass of   with relative abundance 1.1%. The relative atomic mass is calculated below.

RAM=11.868 + 0.143036905

RAM=12.011036905amu