chapter 12 homework
This looks difficult at first, but let's think about what we know. We can calculate how many Ra-226 atoms there are in a gram of Ra. Each alpha particle changes A by four and Z by two. Each emission of an electron signifies the decay of a neutron to a proton, increasing Z by one. leaving A constant.
http://web.mst.edu/~sparlin/phys107/homework/chapter12.pdf