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?
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
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.