For what percentage of time has life existed on earth (round to the nearest whole number).

This is the concept of arithmetic. To calculate the percentage of time life has existed we proceed as follows;
According to history:
The age of earth is 4.6 billion years
the age of life is as it appeared from unicellular is 3.8 billion years.
thus the percentage of the above will be:
3.8/4.6 x 100
=0.82609x100
=82.609%