KöMaL 2020 K649

A fast train and a slow train of the same length are travelling on two parallel tracks, in opposite directions. The tracks both pass through a tunnel. The fronts of the two trains arrive at the two entrances of the tunnel simultaneously. It takes 3 seconds for the total length of the fast train to become covered by the tunnel, and it takes 6 seconds for the slow train. The trains meet inside the tunnel 18 seconds after reaching the tunnel. How long do they take to pass each other? At what time after meeting will the full length of the individual trains emerge from the tunnel?