how transmission carry both power frequency(50Hz) and communication frequency (MHz) at a time
This won't normally happen unless you supply an input that is a combination of the two input signals.
However, any point on the line can only have one voltage at a time, so the line carries a resultant of two input waveforms.
A transmission line, as a linear device, can carry many frequencies (like your cable television) without them interfering. You can then tap in and selectively filter for the signals you want. No reason they can't go down to 60 Hz, or even DC.
A MHz signal source/receiver can be capacitively coupled to a power line and see very little of the 60 Hz power.
yup exactly which is the basis for the cursed BPL system, Broadband over Power Line
On a small and loacalised system, I have dealt with home intercom systems in years gone by that injected and recovered audio onto the house mains system. The intercoms could be plugged into any power outlets around the house and would operate
( NOTE ... Not if there was a multiple phase system used in the building)
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