Assume the number of radioactive nucleus is N(t) --- not yet decayed into daugheter product.
Each nucleus has the same probability p (p<1) decayed into daughter product.
It means that dN(t)/dt= -p*N(t) , where dN(t) is the number of nucleus decayed into daughter product at time t.
The solution is N(t)=N(0) e [sup]-pt[/sup], where N(0) is total number of nucleus at time t.

Each box represent a nucleus.
This applet let you set the probability for a nucleus to decay and watch how the number of decayed nuclei (color changed to RED)  increased with time t.

VIEW or DOWNLOAD ejs xml source
then click to translate strings for selected locale.

Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop.