### Brain Teasers

# Troubling Doubling at School

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...

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