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Riddle of the Day

Troubling Doubling at School

In a class, 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 of students I teach? Here's a clue: More than 20; below 32.

Answer

27 students.

The number of boys must be a multiples of 3 (3, 6, 9, 12, ...) so that it can be split in the ratio of 2:1 (no watch:watch).

The number of girls is double the number of boys - 6, 12, 18, 24, etc.

So the total number of students can only be 9, 18, 27, 36, etc.