Debate team probs

Let the number of girls  =  g

Let the number of boys  =  b

b + g  =  20

b  =  20 - g

probability that a girl is chosen  =  $\frac23$( probability that a boy is chosen )

$\frac{g}{20}\,=\,\frac23(\frac{b}{20})\\~\\ \frac{g}{20}\,=\,\frac23(\frac{20-g}{20})\\~\\ g\,=\,\frac23(20-g)\\~\\ \frac32g\,=\,20-g\\~\\ \frac32g+g\,=\,20\\~\\ \frac52g\,=\,20\\~\\ g\,=\,8$

_ _ _ _ _ _