Posted by Taylor on .
The halflife of iodine 125 is 60 days. What fraction of iodine125 would be left after 180 days?
my teacher didn't teach us this and expects us to know it and I don't understand!!!
could you explain it please!?

Chemistry 
DrBob222,
The half life of a material is the time it takes for exactly 1/2 of the material you started with to disintegrate into another material. In this case, the half life is 60 days. Therefore, 1/2 will be left after 60 days, 1/2 of the 1/2 (that is 1/4) will be left after 120 days (a second half life) and 1/2 of the 1/4 (=1/8) will be left after 120 days (the third half life).
The amount left is 1/2^{n} where n is the number of half lives.
1/2^{n} = 1/2^{3} = 1/8