Answer:
100 Radioactive particles
Explanation:
If your great-grandmother measured 400 radioactive particles 100 years ago and today it has 200 radioactive particles, we observe that the half-life of this particles is 100 year.
That means that each 100 years the number of radioactive particles drop to half of it's original number.
400 radioactive particles--100 year--200 radioactive particles --100 years-- 100 radioactive particles
