Troubling Doubling at School
Math brain teasers require computations to solve.
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!
HintThe sum of number plus its double must be a multiple of 3.
Hide
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...
Hide
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! 
Jimbo
Dec 07, 2014
 Good teaser. Nice to see logical solutions that do not necessarily have to resort to algebra. 
ron93
May 05, 2015
 Clever solution 
Eminem
Jul 05, 2015
 27
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. 
catmom
Dec 20, 2015
 Once again I am reminded that math is NOT my strong suit. 
ultimatecub
Dec 20, 2015
 Nice one, I got it straight away. Only cube number of 3 in that range is 27 
Back to Top
 

Follow Braingle!