[Numeracy 510] Re: Monday Math Puzzle

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Chip Burkitt chip.burkitt at orderingchaos.com
Mon Aug 23 09:17:04 EDT 2010

The clock that doesn't run is right most often.

This is a variation on a conundrum from Lewis Carroll, author of Alice
in Wonderland and Alice Through the Looking-Glass. The clock that
doesn't run at all is right twice a day. For example, if the hands are
set at 10:10 (like the hands of clocks in magazine ads), then the clock
will be right at 10:10 AM and again at 10:10 PM every day. The clock
that loses an hour a day, however, may never be right during the week,
depending on the week. Since it loses an hour a day, it takes 12 days
for it to lose 12 hours, at which time it will be right again. Lewis
Carroll introduced the problem by first asking which was more useful a
clock that was right twice a day or one that was right twice a month.
The intuitive answer is that the clock that is right more often is more
useful, but that turns out not to be the case. (Carroll makes a long,
silly argument to show that a stopped clock is just as useful as one
that loses time. You can find it in

On 8/23/2010 6:20 AM, Istas, Brooke wrote:

> *Puzzle Question: *A woman has two grandfather clocks in her house.

> One of the clocks does not run at all, and the other clock always

> loses an hour a day. Which of the clocks will display the correct time

> most often during any given week?

> I hope that you all are enjoying the puzzles that I have been

> posting. I use these as "warm-up the brain" exercises for students as

> they come to class. Then we discuss the answers. It is a great way

> to start class and get the students ready to discuss math!

> What other techniques do you use in your classes to get the students

> ready to engage their brains?

> Have a great day!

> Brooke



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