Two recording devices are set 3600 feet apart, with device at point A to the West of the device at point B. At a point on line between the devices, 200 feet from point B a small amount of explosive is detonated. The recording devices record the time the sound reaches each one. How far directly north of the site B should a second explosion be done so that the measured time difference recorded by the devices is the same as that for the detonation?