[tex]x-the\ rate\ of\ the\ current\\t-time\\\\ \left\{\begin{array}{ccc}(30-x)t=45\\(30+x)t=63\end{array}\right\\ \left\{\begin{array}{ccc}t=\frac{45}{30-x}\\t=\frac{63}{30+x}\end{array}\right\\\\\frac{45}{30-x}=\frac{63}{30+x}\\\\cross\ multiply\\\\63(30-x)=45(30+x)\\1890-63x=1350+45x\\-63x-45x=1350-1890\\-108x=-540\ \ \ \ /:(-108)\\x=5\\\\Answer:The\ rate\ of\ the\ current\ is\ 5\ km/h.[/tex]