The half-life of cobalt-60 is 5.2 years. Find the time it would take for a sample of 0.5 grams to decay to 0.1 gram?
Just over 12 years
The half life is the time taken for the mass of the substance to decrease by a half.
In this case, the amount of substance remaining is 20% of the initial amount (0.1 g out of initial 0.5 g).
Therefore you can say:
Now rearrange for n:
So it would take 2.32 half lives to decay this much, which is 2.32 x 5.2 which is 12.064 years.