If everything else remains constant, then the received signal power is simply an inverse-square function of its distance from the transmitter.
That is, down 6dB when distance is doubled etc.
If distance increases from 16 to 20, then received power decreases by the factor of (16/20)^2.
That's (0.8)^2 = 0.64
New receive power is 5x0.64 = 3.2 nW
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Here's how I do it at my job:
Initial RSL = 5nW ~ - 53 dBm
loss = 20log(20/16)= 20log(1.25)~1.94dB
New RSL = - 54.94 dBm or ~ 3.2 nW .
