A riverside warehouse has two open doors as shown in Figure. Its walls are lined with sound-absorbing material. A boat on the river sounds its horn. To person A the sound is loud and clear. To person B the sound barely audible. The principal wavelength of the sound waves is `2 m`. Assuming person B is at the positon of the first minimum, determine the distance `Ab = 20 m` and the distance of A from the doors is `150 m.