For the one-decay solution :
the equation indicates that the decay constant  has units of , and can thus also be represented as 1/, where  is a characteristic time of the process called the time constant.
In a radioactive decay process, this time constant is also the mean lifetime for decaying atoms. Each atom "lives" for a finite amount of time before it decays, and it may be shown that this mean lifetime is the arithmetic mean of all the atoms' lifetimes, and that it is , which again is related to the decay constant as follows:
This form is also true for two-decay processes simultaneously , inserting the equivalent values of decay constants (as given above)
into the decay solution leads to: