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!

 

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...