Ageologist finds an old rock and wants to determine its age using rubidium-strontium dating. this is possible because 87rb, which has a half-life of 4.75×1010 y, undergoes β − decay and becomes 87sr. the geologist determines that the ratio of 87sr to 87rb is 0.183. assuming there was no 87sr in the rock when it was formed, determine the age of the rock.

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.