Answer
In the time it takes the current to make one oscillation, the EM wave travels a distance of 5000 km.
The wavelength of a 60-Hz EM wave is 1.2 times the distance between Boston and Los Angeles.
Work Step by Step
If the current oscillates at a frequency of 60 Hz, then the current oscillates 60 times per second. The time for one oscillation is $\frac{1}{60}~s$
We can find the distance the EM wave travels in this time:
$d = (300,000~km/s)(\frac{1}{60}~s) = 5000~km$
In the time it takes the current to make one oscillation, the EM wave travels a distance of 5000 km.
We can compare this length to the distance between Boston and Los Angeles:
$\frac{5000~km}{4200~km} = 1.2$
The wavelength of a 60-Hz EM wave is 1.2 times the distance between Boston and Los Angeles.
