Braingle: 'Troubling Doubling at School' Brain Teaser

Troubling Doubling at School

Math brain teasers require computations to solve.

Puzzle ID:	#49613
Fun:	(2.94)
Difficulty:	(1.81)
Category:	Math
Submitted By:	dalfamnest

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!

Show Hint

Answer

27

Solution:
The number of boys must be a multiple of 3 (3, 6, 9...) so that it can be split in the ratio of 2:1 (no watch:watch).

The number of girls is double the no. of boys (6, 12, 18...)

So the totals can only be 9, 18, 27...
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Comments

kittygirl19 Sep 27, 2012	First!
Marple Oct 02, 2012	Second!
RRAMMOHAN Oct 07, 2012	A bit tricky, but got it! Nice and simple explanation for the solution.
Babe 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!
jaycr Dec 20, 2012	Hmm, this could be interesting...
HABS2933 Dec 20, 2012	Fairly easy, but still fun. Though not for everyone, I do enjoy math (and most other subjects for that matter).
HABS2933 Dec 20, 2012	Fairly easy, but still fun. Though not for everyone, I do enjoy math (and most other subjects for that matter).
iggy39 Dec 20, 2012	Me too Babe, I don't have the brain to work these out.
dsjt 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?
dalfamnest 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!