Radiologically, half lives describe the amount of time it takes for half the amount of the radioactive substance to decay. If you had 100g of a radioactive substance with a half life of one week, in seven days, only 50g would remain, and after another week, only 25g.
Fifteen days contains three half lives since the half life is 5 days long. Divide the starting amount by 2 for each half life that passes.
600g - 300g (1 HL) - 150g (2 HL) - 75g (3 HL)
So, only 75g of Bismuth-210 would remain after 15 days.
