### Brain Teasers

# Troubling Doubling at School

Math
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!

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!

### Hint

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

27Solution:

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 Hint Show Hint Hide Answer Show Answer

## What Next?

**Solve a Similar Brain Teaser...**

Or, get a random brain teaser.

If you become a registered user you can vote on this brain teaser, keep track of

which ones you have seen, and even make your own.

## Comments

First!

Second!

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

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

Hmm, this could be interesting...

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

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

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?

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!

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

Happy New Year to all - keep enjoying this site!

Good teaser. Nice to see logical solutions that do not necessarily have to resort to algebra.

Clever solution

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.

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.

Once again I am reminded that math is NOT my strong suit.

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

Everyone is assuming that the mids actually wear watches.

If none of the kids wear watches at all, then double 0 is 0.

So if it's between 20 and 32 then any multiple of 3 will work. For instance 7 girls and 14 boys makes 21 students, or 24 students (8 and 16) or 27 students (9 and 1.

Kids these days dont wear watches anyways, with their smartphones!

If none of the kids wear watches at all, then double 0 is 0.

So if it's between 20 and 32 then any multiple of 3 will work. For instance 7 girls and 14 boys makes 21 students, or 24 students (8 and 16) or 27 students (9 and 1.

Kids these days dont wear watches anyways, with their smartphones!

## Follow Braingle!