In a population of 22,000,000.00What is the probability of three people meeting randomly who lived within three houses from each other 43 years ago?

Question

1973 - Perth West Australia,  population 900,000: age 12 :- I live in an apartment block of 40 units. 

1995 - Perth West Australia, population of 1,800,000: age 33 :- meet husband in a blind date. While married we discover he lived in that apartment block 21 years previously. 
2015 - Perth Australia, population 2,200,000.00: Australia Population 23,000,000: age 54 :-  
I receive a phone call from a stranger "john" who lives on the other side of the country in Townsville, Queensland.
He learned we share a mutual friend in Mexico - where I visited 9 months prior to him calling. 
3 months later,  he and his wife visit us in Perth and we discover they also lived in those apartments in 1973 - 43 years ago 
(They had moved state 14 years ago)

What is the probability this would happen? 

 

Answer 1

The problem can only be estimated by making assumptions about the total number of people who were living in the three house radius at the time and how people make contact with each other, when they make contact and how. But the real wild card is the time frame in which they meet again. The odds are beyond astronomical if you want them to meet at the same time, such as at a party, anyplace in the world. Your criteria also allows them to meet at any time even indirectly by telephone or the internet. 

 

So you problem cannot be solved precisely by typical math probability methods without modeling how any three people might meet in any manner, anywhere in the world, even considering the population the population of Perth. It is not that simple. It would be possible to compute if everyone stayed in Perth and just ran around randomly without discussing everyplace that they had lived, and made contact at the same time 43 years later. The class of problem is called Brownian Motion.

 

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