Stuff about the earth and tectonic plates, and about space and stuff.
Answer:
is the activity, measured in of a 50 mg sample of 90-Sr.
Explanation:
Half life of the 90-Sr ,
Activity coefficient of the 90-Sr = 

Mass of 90-Sr = 50 mg = 0.050 g
Molecular mass of 90-Sr = 90 g/mol
Moles of 90-Sr =
Number of atom in 0.0005555 moles of 90-Sr:


1 year = 
Activity measured in atoms per seconds:
= Number of atoms × 


A noble metal is a metal that resists oxidation and does not corrode (i.e: rust). Silver and platinum are great examples since they don't rust