Half-Life Period (T1/2 or t1/2)
The half-life period of a radioelement is defined, as the time required by a given amount of the element to decay to one-half of its initial value.
Now since l is a constant, we can conclude that half-life period of a particular radioelement is independent of the amount of the radioelement. In other words, whatever might be the amount of the radioactive element present at a time, it will always decompose to its half at the end of one half-life period.
Let the initial amount of a radioactive substance be 
Amount of radioactive substance left after n half-life periods
Total time T 
where n is a whole number.