In the parts that follow, use the following abbreviations for time: Measure of time Units of time
minutes min
hours h
days d
years yr

Required:
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. counts after 25.8 minutes , what is the half-life of this substance?
b. An unknown radioactive substance has a half-life of 3.20 hours . If 17.1 g of the substance is currently present, what mass A0 was present 8.00 hours ago?