Ans: $78$
Let $x$ and $y$ be the original number of boys and girls in the summer camp respectively.
Let $x$ and $y$ be the original number of boys and girls in the summer camp respectively.
$\left\{ \begin{array}{ll}
\dfrac{x}{y} = \dfrac{7}{6} & \ldots \unicode{x2460} \\
x-17 = y-4 & \ldots \unicode{x2461}
\end{array}\right.$
From $\unicode{x2461}$, we have
$\begin{array}{rcl}
x-17 & = & y-4 \\
x & = & y+13~\ldots \unicode{x2462}
\end{array}$
Sub. $\unicode{x2462}$ into $\unicode{x2460}$, we have
$\begin{array}{rcl}
\dfrac{y+13}{y} & = & \dfrac{7}{6} \\
6y+78 & = & 7y \\
y=78
\end{array}$
Therefore, there are $78$ girls originally in the summer camp.
