Beanium Boo Boo

Recently in class, we did a "Beanium Lab" (Beanium is a fictional element created for this learning experience). In order to paricipate in the lab, we had to pass a 1 question Pre-Lab quiz. I was the only student in my hour who didn't answer correctly. The question was over calculating % abundances and average atomic mass. I completely blanked, guessed an answer, and failed. I need to revisit these equations in order to be successful in this unit in the future.