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?

considering the question as per the attachment

here we can see that the radon nuclide has 222 as the atomic mass number and 86 as the atomic number.

when it undergoes nuclear fission it gives polonium which has atomic mass number as 218 and atomic number will be 84 along with a helium nucleus which has 4 atomic mass number and 2 as its atomic number.

222 = 218 + 4 (atomic mass number)

86 = ? + 2; therefore ? = 86-2 = 84.

the atomic mass number of polonium will be 84.

the pseudoscience are like beliefs, theories, or like practices that are considered scientific