A building has two open doors facing the bay. The walls of the building absorb sound well, so multiple reflections can be ignored. Two people are standing a distance L = 150 m from the wall with the open doors, as pictured. Person A is directly opposite the midpoint between the two doors while person B is 20 m to his side. The boat sounds its horn and person A hears it loud and clear while person B can barely hear it. The sound waves have a wavelength of 3.0 m. If person B is at the first minimum, find the distance d between the door centers.