[Tutorial: Average atomic mass calculation in amu] This question will walk you through the steps of calculating an element's average atomic mass. Consider boron, which occurs in nature as a mixture of two stable isotopes. Step 2b: Design a problem-solving strategy. This problem will be solved by calculating a weighted arithmetic mean. The relative abundance of boron-11 in decimal form is 0.8010 Calculate the relative isotopic mass in amu of the boron-11 with a natural abundance of 80.10% with an exact mass of 11.009 amu.