Question

naturally occurring rubidium consists of about 73.2% rubidium-85 (atomic mass 84.9 u) and 27.8% rubidium-87 (atomic mass 86.9 u). calculate the average atomic mass of rubidium rounding your answer to 0.1 u

Answer 1

The answer based on the question would be 86.3.  How that answer was derived is attached below.

Answer 2

You can find the average by adding rubidium-85's and -87's weights then dividing the sum by two. I'm fairly certain it's 85.9 u but I would check my work