The calculated distance (air line) between Seongnam-si and Pyeongtaek is approximately 31 miles respectively 50 kilometers.
By car or train, the actual journey to Pyeongtaek is certainly longer, as only the direct route (as the crow flies) between Seongnam-si and Pyeongtaek has been calculated here.
Air line (approximately)
50 kilometers
27 nautical miles