JAMB - Physics (1999 - No. 44)

The frequency f = ^V/_λ,
Where λ = wavelength
From the above, the distance OA = one wavelength; AB = one wave length and BC = one wavelength.
=> OC = 3 wavelength = 3λ
∴3λ = 0.3M
λ = ^0.3/₃
= 0.1m
∴ f = ^V/_λ = (3.0 x 10⁸) / 0.1
= 3.0 x 10⁹Hz