To step 110 V ac down to 20 V ac, the turns ratio must be

Vp/Vs = Np/Ns
Vs = (Ns/Np)Vp
We required turns ratio i.e., (Ns/Np)
= Vs/Vp
= 20/110
= 0.181818182
[Note: p - primary; s - secondary]
For step down transfer, the turns ratio will be less than 1.
For simulation: Step-Down Transformer Circuit Simulator.