If the half-life is 15 days and the initial count rate is 200 counts per second, what would the count rate be after 45 days?

Because 45=3x15, 45 days is three half-lives, so the initial count rate of 200 would be 25 because, 200/2 = 100, 100/2= 50 and 50/2 = 25 counts per second