How do you determine the total mass of an original 28.0-milligram sample of Po-210 that remains unchanged after 414 days?
Given the 138.4 day half-life of the radioisotope
Given the boundary conditions of the problem it was SPECIFIED that the mass of
And so your polonium isotope has experienced 3 half-lives.........And there is thus
Note that I think this was the first radioisotope isolated by Marie Curie who named it after her native Poland.