chemistry
posted by cam .
caffeine has a halflife of 4.6 hours in the human body.. How long would it take 125 milligrams tritium to decay to 22 milligrams?

Pray tell what does caffeine have to do with tritium?
For tritium,
k = 0.693/t_{1/2}
You will need to look up the halflife of tritium unless that is the "caffeine" halflife.
Substitute k into the below equation.
ln(No/N) = kt
Nl = 125
N = 22
k = from above calculation.
t = time. Solve for this. Note that the unit for time and the unit for halflife must be the same.