Puzzle ZHNZ 

Workings

Hint
A little bit of algebra might help.

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 Billy is a boy):

              B - 1 = G

and similarly:

        2 x (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.