Formula for radioactive decay
A = A0 * (1/2)(t/h)
where
- A = final amount
- A0 = initial amount
- t = time
- h = half life of element
With a little algebra, this formula can be transformed to give an equation to find the time (t), given the starting and ending amount.
ln( A/A0 )
t = h ------------
ln( 1/2 )
To determine time required for a radioactive element with a half-life of 12.3 years (Tritium) to lose 25% of its mass (i.e., 75% remains)
ln(1/0.75)/ln(1/0.5)*12.3 = 5.1