#1
speed of sound = 340 m/s
frequency = 829 Hz
wavelength of sound = speed / frequency

Now for constructive interference we know that
path difference of two waves = integral multiple of wavelength


if N = 3

#2
speed of sound = 344 m/s
frequency range = 252 Hz - 665 Hz
wavelength of sound = speed / frequency

Now for constructive interference we know that
path difference of two waves = integral multiple of wavelength


Now we will have


by rearranging above terms we have

