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Possibily this is a simple question, can any one guess how many people have ever lived on earth? Is there any way to calculate it ?
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106.5 Billion ! (As per a calculation done in 2002). 

Another calculation done in 1998 showed 96,100,000,000 people have lived on the earth. Solution :- Keyfitz assumed that exponential growth occured in various historical time intervals and he used the following formula to calculate it , the total person-years from A to B is [P(B)-P(A)](B-A)/{Ln[P(B)]-Ln[P(A)]}. The above formula is applied to a system of time intervals from 1,000,000 BC to the present. The sum of the total person-years for all these intervals gives the total person-years for all human life. By dividing by the average lifespan(in Keyfitz's work average lifespan is taken as 25 years), we can obtain an estimate of how many people have ever lived.

You can refer to,
http://www.prb.org/Articles/2002/HowManyPeopleHaveEverLivedonEarth.aspx
http://www.math.hawaii.edu/~ramsey/People.html

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