Two loudspeakers are placed side by side a distance dd $=4\mathrm{.}00$ mm apart. A listener observes maximum constructive interference while standing in front of the loudspeakers, equidistant from both of them. The distance from the listener to the point halfway between the speakers is ll $=5\mathrm{.}00$ mm $.$

One of the loudspeakers is then moved directly away from the other. Once the speaker is moved a distance rr $=60\mathrm{.}0$ cmcm from its original position, the listener, who is not moving, observes destructive interference for the first time.

Find the speed of sound vv in the air if both speakers emit a tone of frequency $700$ HzHz $.$

How far $($r$)$ from its original location should the speaker be moved for the listener to experience constructive interference for the second time?