This is known as the integrated rate law
equation. No is
the original amount of sample while N is the amount remaining at time t. This equation allows us to determine the decay rate of
a sample at any given time AND to predict how much of a given
sample will remain at any given time. It also shows that the rate
behavior is logarithmic in nature. 
Nuclear scientists often refer to the decay rate in terms of a quantity called half-life. This is simply the time it takes for half of a sample to decay into something else. The half-life can be related to the rate constant using the integrated rate law and recognizing that the ratio No/N will be 2/1 when t is the half-life:
