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.