Answer
Boys $=4$.
Girls $=3$.
Work Step by Step
Let the number of boys $=x$.
and the number of girls $=y$.
For a boy:
Number of brothers $=x-1$.
Number of sisters $=y$.
From the question we have
$\Rightarrow y=x-1$ ...... (1)
For a girl:
Number of brothers $=x$.
Number of sisters $=y-1$.
From the question we have
$\Rightarrow x=2(y-1)$
Simplify.
$\Rightarrow x=2y-2$ ...... (2)
Substitute the value of $y$ from equation (1) to equation (2).
$\Rightarrow x=2(x-1)-2$
Apply distributive property.
$\Rightarrow x=2x-2-2$
Add like terms.
$\Rightarrow x=2x-4$
Add $-x+4$ to both sides.
$\Rightarrow x-x+4=2x-4-x+4$
Simplify.
$\Rightarrow 4=x$
Substitute the value of $x$ into equation (1).
$\Rightarrow y=4-1$
Add like terms.
$\Rightarrow y=3$.
