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[Phys-l] C & C Trajectories

Click and Clack offered a version of the following puzzler
this morning.

A family of four drove up the west coast 400 miles on Saturday,
starting at 8 a.m. They arrived at their destination at 4 p.m.
Three of them returned home the following day by the same route,
in the same car, starting at 11 a.m. Sunday.

The question: what is the probability that the car passed the
same spot at the same time of day, going both ways?
I was surprised that the answer was not immediately obvious.
To me at least.
The idea of space time diagrams came to mind.

Brian Whatcott Altus OK Eureka!