Half-Life of Decay and Average Life

Half-Life of Decay:

It is defined as the time after which number of atom left will become half of initial number of atom, Then from equation

Average Life:

It is defined as the total life time of all the atoms of the element and dividing it by the total number of atoms present initially in the sample of the element. 

Or

It is the average of the lives of all the atoms in a sample.