A speaker fixed to a moving platform moves toward a wall, emitting a steady sound with a frequency of $240$ HzHz $.$ A person on the platform right next to the speaker detects the sound waves reflected off the wall and those emitted by the speaker.

Part A

How fast should the platform move, vpvp$,$ for the person to detect a beat frequency of $7\mathrm{.}00$ HzHz $?$

Take the speed of sound to be $344$ m$/$sm$/$s$.$