The astronaut then measures the abundance of strontium on the new planet, obtaining the following results: isotope abundance (%) mass (amu) 86sr 9.46 85.91 87sr 7.00 86.91 88sr 83.54 87.91 what is the atomic mass of strontium for this planet? express your answer to two decimal places, and include the appropriate units.

The process for knowing all of the aboce is the following:
Atomic mass = (0.0946x85.91) + (0.07x86.91) + (0.8354x87.91) = 87.6508 amu
The answer is that having in mind that Atomic mass is equal to the average number allowing for the relative abundances of different isotopes. That is calaculated by using the following principle or formula:  ΣzₓAₓ 
zₓ is the relative abundance of the isotope and Aₓ is the atomic mass of the isotope.