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.