jmc

To make the distance between exit $47$ and exit $48$ as long as possible, we want to make the distance between any other two consecutive exits as short as possible (that is, $6$ km). There are nine distances between exits from exit $41$ to exit $50$; eight that we want to make as short as possible and one that we want to make as long as possible. Thus, the longest possible distance is $100-8\cdot 6=\boxed{52}$ km.