Answer:
The answer to your question is: 87.712 amu
Step-by-step explanation:
Strontium isotopes 1 mass 84 amu abundance 0.5%
2 mass 86 amu abundance 9.9 %
3 mass 87 amu abundance 7%
4 mass 88 amu abundance 82.6%
average atomic mass = (84 x 0.005) + (86 x 0.099) + (87 x 0.07) + (88x0.826)
average atomic mass = 0.42 + 8.514 + 6.09 + 72.688
average atomic mass = 87.712 amu