By definition 'half time' is the time in which a mass of material subject to decay [Uranium for example...] is halved. The decay law is then...
... where is the 'half time' and the mass of material at the time ...
Now that we have , we can set up our equation.If you have 35 grams of uranium-232 to begin with, the equation that describes the radioactive decay is
The decay equation has the form , where is the initial amount of the substance.
Thus, our decay equation is