Thus, the mixture taken as a whole will not decay by halves.
A biological half-life or elimination half-life is the time it takes for a substance (drug, radioactive nuclide, or other) to lose one-half of its pharmacologic, physiologic, or radiological activity.
In a medical context, the half-life may also describe the time that it takes for the concentration of a substance in blood plasma to reach one-half of its steady-state value (the "plasma half-life").
The relationship between the biological and plasma half-lives of a substance can be complex, due to factors including accumulation in tissues, active metabolites, and receptor interactions.
Mathematically, the sum of two exponential functions is not a single exponential function.
A common example of such a situation is the waste of nuclear power stations, which is a mix of substances with vastly different half-lives.
In other words, the probability of a radioactive atom decaying within its half-life is 50%.