Arjun is standing on an old building and there is a railway track beside, on which a train travels at a speed of 72 kmph. When the train passes by the building, Arjun tries to jump inside the train. He somehow managed to enter the train through the last door of the last bogie. After jumping in, he runs towards the other end of the train at a speed of 36 kmph. When he reaches a distance of 300 m from the building the building exploded. After how many seconds since Arjun's jump did the building explode?