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Puzzle Details:

Daniel has the same number of brothers as sisters.

His sister, Laura, has twice as many brothers as she has sisters.

How many boys and girls are in the family?

[Ref: ZHNZ] © Kevin Stone

Answer: There are four boys and three girls.

If we call the number of boys B, and girls G...

From the first statement (we take one off as Daniel is a boy):

B - 1 = G

and similarly:

2 * (G - 1) = B

Substitute G from the first equation into the second equation to give:

2 x ((B - 1) - 1) = B

2 x (B - 1) - 2 = B

2 x B - 2 - 2 = B

2 x B - 4 = B

B = 4

which means that B = 4 and hence G = 3. QED.

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