CN carrier to noise and distant

I have to answer that problem:

A radio receiver 2km distant from a radio transmitter has a carrier to noise ratio (CN) of 40dB. Assuming that noise power remains constant irrespective of position, determine the CN ratio at a distance 10km from the transmitter using:
– inverse law of power propagation
– fourth power law of propagation

I dont expect so that you solve that. I just need to know an idea of solution

thx in advance