need answers for all three :)

You have a 200 gram sample of Radon. Its half life is 4 days. How much will remain unchanged after 24 days? A sample of I-131 has a half life of 5 hours. If the original sample was 100 grams, how much of the sample would be left after 4 half lives? How much time will have gone by after 4 half lives? A sample of Sriontium-90 decays to 50 grams in 3 half lives. How much was the original sample