Subject: Can a decay curve of a radioactive substance ever reach 0?

I am wondering if the decay curve of a Radioactive material can ever reach 0
because we recently had to find out what the counting rate of a unknown radioactive
substance would be after 1 day and we had the data for the start and some times.
 When I calculated what it would be I got 1.61 * 10^-11. The answer that my
teacher supplied as the correct answer was 0.
The data that I had was
Time (minutes)    Net counting Rate (counts / Minute)
0                 171
20                110
40                70
60                49
80                31

To get my anser I figure out at what rate it drops per hour by doing
49/171 = .28655
Then I knew that in order to get how much it would drop in 24 hours it would be
171 * .28655^24 = 1.61 * 10^-11 counts / minute

My teacher was not able to give calculation on how she reached her answer she
just said that it wouldnt have any more particles to give off, which I believe
is not mathematically possible since it must follow a decay curve that starts
with two non zero numbers, but since we are dealing with a finite ammount of a
substance I am not sure how it would work.  What I am wondering is: Can a decay
curve of a radioactive substance ever reach 0?  Who is correct using that data
that we were given? Did I calculate the data the proper way?
Thank you,
Adam

Re: Can a decay curve of a radioactive substance ever reach 0?