How long does it take a 100.0 g sample of As-81 to decay to 6.25g?

How long does it take a180 g sample of Au-198 to decay to 1/8 its original mass?

What is the half-life of a radioactive isotope if a 500 g sample decays to 62.5g in 24.3 hours?

How old is a bone if it presently contains 0.3125 g of C-14, but it was estimated to have originally contained 80.0 g of C-14?

Thallium-208 has a half-life of 3.053 minutes. How long will it take for 120.0 grams to decay to 7.50 grams?