Question 1030584
Hi there,
The half-life of a radioactive isotope is 
the time it takes for a quantity of the 
isotope to be reduced to half its initial Activity. 
200 gms  - 100 gms 1 half life
100 gms  -  50 gms  2 half lives
50 gms   - 25 gms  3 half lives
25 gms   -  12.5 gms  4 half lives
12.5 gms  - 6.25 gms  5 half lives
= 6 gms.
Hope this helps :-)