Answer: 20.2
Explanation:
Mass of isotope 1 = 20
% abundance of isotope 1 = 90.5% = 
Mass of isotope 2 = 22
% abundance of isotope 2 = 8.0% = 
Mass of isotope 3 = 23
% abundance of isotope 3 = 1.5% = 
Formula used for average atomic mass of an element :
![A=\sum[(20\times 0.905+(22\times 0.08)+(23\times 0.015]](https://tex.z-dn.net/?f=A%3D%5Csum%5B%2820%5Ctimes%200.905%2B%2822%5Ctimes%200.08%29%2B%2823%5Ctimes%200.015%5D)
Therefore, the average atomic mass of the element is 20.2.
I'm assuming that you are asking a general question because you did not include an example.
The limiting reagent is the item in the reactants (reagents) that will run out first. This is because it limits what the reaction can produce, essentially causing the leftover elements/compounds to just sit there.
Sometimes negative sometimes positive, your answer is B!
Californium which is element 98, and americium:) those are a couple!! Hope that helps
