The Lost Sheep Challenge

Andy Bradford amb-plug at
Fri Sep 28 16:16:11 MDT 2007

Thus said "Sasha Pachev" on Fri, 28 Sep 2007 16:07:52 MDT:

> You have 100 sheep  in a flock and they are all  numbered. Each has an
> identification tag with a number - 1 through 100. One of them is lost.
> Other sheep are  scattered over the pasture and cannot  be examined in
> sequential order  of their numbers. Come  up with a method  that would
> allow you  to quickly  identify the  lost sheep. The  use of  a simple
> arithmetical calculator is allowed.

Hopefully its  an HP  with RPN,  but that might  violate your  roll call
rule. :-)

Just add up the  numbers 1--100 for a total of  5050. Then subtract each
sheeps number  as you encounter  it in  the field (hopefully  they don't
roam back  and you  double count  them). The end  result is  the missing

This probably isn't the best algorithm, but it seems it would work, no?

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