If we know how much time an isotope has been left for, we can work out what the half life is for it. Example:
If we started with 120 g of Protoactinium-234, and had 1.88 g after 7 minutes, work out a value for its half life.
The sequence would go: 120>60>30>15>7.5>3.75>1.88 which is 6 half lives.
So one half life is 7 minutes / 6 = 1.17 minutes.
Of course, like most things in physics, a graph is the best way of showing this.