60 x 5 = 300 days
So five half-lifes = 300 days
Starting amount = 500mg
Amount after 1st half-life = 250 mg
Amount after 2nd half-life = 125 mg
Amount after 3rd half-life = 62.5 mg
Amount after 4th half-life = 31.25 mg
Amount after 5th half-life = 15.625 mg
Hence after 300 days, the amount of iodine-125 will be 15.625 mg out of 500 mg.