Are we assuming that you mean you can sustain 20 MPH with no wind, or is that top speed?

The forces limiting your speed are:

1) air resistance
2) tire rolling resistance
3) mechanical friction

At speeds above around 15 mph, the second two become less and less important as your speed increases. Air resistance increases exponentially as the combined sum of your speed + the wind speed increases, while the other two increase more or less linearly with your speed.

So, if you ride at 1 MPH into a 20 MPH headwind, the force you must overcome from air resistance is the same as the force of riding 21 MPH with no wind, but the mechanical friction and rolling resistance would be about 1/20th as much. This difference determines how fast you can go. It won't be very fast.

With a 20 MPH tailwind, the air resistance from riding at 20 MPH is nearly canceled (though not exactly), so you can go much faster. Still you won't be able to go 40 MPH because the other two forces are still increasing as you gain speed.

All of this assumes we're talking about riding on flat surfaces. Gravity, of course, throws in additional wrinkles.

There are also undoubtedly psychological forces that come into play as riding into a headwind can very quickly drain your will to live.