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Troubling Doubling at School

Math brain teasers require computations to solve.


Puzzle ID:#49613
Fun:*** (2.88)
Difficulty:** (1.63)
Submitted By:dalfamnestAnz******




Read the little poem and answer its question if you can.

The number of girls who do wear a watch
is double the number who don't.
But the number of boys who do not wear a watch
is double the number who do.

If I tell you the number of girls in my class
is double the number of boys,
Can you tell me the number I teach? Here's a clue:
More than 20; below 32!

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The sum of number plus its double must be a multiple of 3.

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Sep 27, 2012

Oct 02, 2012

Oct 07, 2012

A bit tricky, but got it! Nice and simple explanation for the solution.
Dec 20, 2012

Fourth! I do not even tempt to figure these out. Not good at math, plus it is not myfavorite to do! Sorry!
Dec 20, 2012

Hmm, this could be interesting...
Dec 20, 2012

Fairly easy, but still fun. Though not for everyone, I do enjoy math (and most other subjects for that matter).
Dec 20, 2012

Fairly easy, but still fun. Though not for everyone, I do enjoy math (and most other subjects for that matter).
Dec 20, 2012

Me too Babe, I don't have the brain to work these out.
Dec 21, 2012

dalfamnest, can you please explain how you know that the boys must be a multiple of three so that those of us who did not find it obvious can learn?
Jan 06, 2013

Hi dsjt...and others. Because the number who 'do' is double the number who 'don't', there must be TWO who 'do' for every ONE who 'doesn't'. So you could arrange them in groups of THREE - 2 who 'do' with 1 who 'doesn't'. The total number must therefore be a multiple of THREE.

I hope that helps - sorry for the delay replying ... I've had a holiday!

Happy New Year to all - keep enjoying this site!
Dec 07, 2014

Good teaser. Nice to see logical solutions that do not necessarily have to resort to algebra.
May 05, 2015

Clever solution
Jul 05, 2015

Its quite easy.
Thanks to your clue i got tbe answer quickly

Total girls = 3x
Total boys= 6x (since there are double the girls)
=> 3x+6x=y
=> x= y/9

Possible solution is 27.
Dec 20, 2015

Once again I am reminded that math is NOT my strong suit.
Dec 20, 2015

Nice one, I got it straight away. Only cube number of 3 in that range is 27

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