How many days are required for 200. grams of radon-222 to decay to 50.0 grams?

(1) 1.91 days (3) 7.64 days (2) 3.82 days (4) 11.5 days

The answer is (3) 7.64 days. The remaining mass of radon-222 is one fourth of the original mass, so two half lives have elapsed (because one half time one half equals one fourth). Each half life of radon-222 is about 3.82 days, so the total time elapsed is 3.82 * 2 = 7.64 days