A riverside warehouse has several small doors facing the river. Two of these doors are open as shown in Figure $\mathrm{P} 27.17$ The walls of the warehouse are lined with sound-absorbing material. Two people stand at a distance $L=150 \mathrm{m}$ from the wall with the open doors. Person A stands along a line passing through the midpoint between the open doors, and person $\mathrm{B}$ stands a distance $y=20 \mathrm{m}$ to his side. A boat on the river sounds its horn. To person $\mathrm{A}$, the sound is loud and clear. To person $\mathrm{B}$, the sound is barely audible. The principal wavelength of the sound waves is $3.00 \mathrm{m}$. Assuming person $\mathrm{B}$ is at the position of the first minimum, determine the distance $d$ between the doors, center to center.