Stronitum-90 has a half-life of 29 years. if a site held 2000 kg of this isotope, approximately what mass of strontium-90 would be left after 116 years?

A half life of 29 years means that after 29 years, only half of this isotope would be left due to radioactive decay. If we start of with 2000g of it and leave it for 116 years, it would undergo 4 "half=lives". 116/29 = 4. So after the first half life, we would have 2000g/2 = 1000g left. After the second, we would have 500g left, after the third, 250g so after 4 half lives or 116 years, there would be 125 g of Strontium-90 left at the site.