How much the oscillation phases differ at two points in the air spaced at a distance of 246 m

How much the oscillation phases differ at two points in the air spaced at a distance of 246 m, if the oscillation particle of the generator is 15 MHz, the propagation speed is 3 * 10 ^ 8.

We translate the values from given to SI:
ν = 15 MHz = 0.015 Hz.
The phase difference of oscillations located at a distance S from each other is determined from the expression:
∆φ = 2π * S / λ
The wavelength is equal to the distance over which the waves propagate in a time equal to the oscillation period of the source.
λ = v * T, where v is the propagation velocity, T is the wave period.
The oscillation period depends on the frequency according to the law:
T = 1 / ν
Then:
λ = v * T = v / ν
substitute this into the phase difference:
∆φ = 2π * S / (v / ν) = 2π * S * ν / v
Substitute the numerical values and determine the phase difference:
∆φ = 2π * S * ν / v = 2π * 246 * 0.015 / 3 * 10 ^ 8 = 7.7 * 10 ^ -8
Answer: the phase difference is 7.7 * 10 ^ -8.