Part A. You are using a Geiger counter to measure the activity of a radioactive substance over the course of several minutes. If the reading of 400. counts has diminished to 100. \rm counts after 93.1minutes , what is the half-life of this substance? Express your answer with the appropriate units. t_1/2 = _____ Part B. An unknown radioactive substance has a half-life of 3.20hours . If 37.0g of the substance is currently present, what mass A_0 was present 8.00 hours ago? Express your answer with the appropriate units. A_0 = _______ Part C Americium-241 is used in some smoke detectors. It is an alpha emitter with a half-life of 432 \rm years. How long will it take in years for 43.0\% of an Am-241 sample to decay? Express your answer with the appropriate units