Find vilocity of sound in a gas when the 2 waves of wavelenth 0.8 and 0.81 metre produce 4 beat per second

The beat frequency (4 beat per second = 4 Hz ) corresponds to the difference between the frequencies of the two waves:
(1)

The frequency of the first wave can be written as

where v is the speed of sound in the gas and  is the wavelength of the first wave, while the frequency of the second wave is

where  is the wavelength of the second wave. If we substitute into the first equation (1), we find


and if we re-arrange it, we can find the velocity of the sound in the gas:

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