Equivalent noise resistance(s) - 2 stage amplifier
The impedances need to match for maximum power transfer, but you probably don't care so much about that for the first stage output. But the input impedance of the second stage should probably be higher than the output impedance of the first stage, or the second stage will load the first stage and limit your operating range.

An ideal amplifier has infinite input impedance and zero output impedance, or at least a reasonable facsimile of those conditions. On the other hand, if your first stage tries to drive 10V through an output impedance is 1k and into an input impedance of 10 ohms, then the second stage will receive $$\frac{10V\cdot10}{(1k+10)ohms} = 0.1V.$$  

In your case, you're driving through 20k into 50k. So your second stage will see $$\frac{50}{(50+20)}$$ or 5/7 of whatever the first stage puts out.

Sorry I don't know Mathjax so ugly equations. BTW, what is this textbook you're using?