[extropy-chat] what is probability?

gts gts_2000 at yahoo.com
Wed Jan 17 22:20:04 UTC 2007

Another pesky Principle of Indifference problem...

A train leaves at noon to travel a distance of 300km. It travels at a  
constant speed of between 100km/h and 300km/h. What is the probability  
that it arrives before 2pm?

At least two solutions with different answers:

1. For the train to arrive before 2pm, its velocity must be greater than  
or equal to 150km/h. By the principle of indifference, all velocities  
between 100 and 300 km/h are equally likely. The probability of it being  
greater than or equal to 150km/h such that the train arrives before 2pm is  
therefore 3/4.

2. The train must arrive between 1pm and 3pm. 2pm is half way between  
these two. By the principle of indifference, the train is as likely to  
arrive before 2pm as after 2pm. So the probability must be 1/2.

What would Scooby Do?


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