Time in producing 15 crests and 15 troughs = 3 sec Now as we know wavelength is the distance traced between two consecutive nodes which ultimately covers one crest & a trough hence sound covers a distance of 15λ in 3 seconds So in this way we can calculate time to cover a distance of λ, Time period ie T = 0.2 sec so frequency, ν = 1/T Also given when the second crest is produced, the first crest is 2 cm away from the source which means wavelength, λ = 2 cm or 0.02 m