Autofill text box based on radio button choice

We have tried a variety of methods and javascript and absolutely nothing will work. Ideas and help are greatly appreciated!

Here are two codes we tried with no luck:

var A = this.getfield("Group16").value;

var Y = this.getfield("Text21").value;

var Z = this.getfield("Text22").value;

if (A=="Choice1")

document.getElementById("Text27").value = Y;

else document.getElementById("Text27").value = Z;

document.getElementById("Text27").value=this.getElementById("Group16").value;

#2:

var S = this.getfield("Text21").value;

var T = this.getfield("Text26").value;

{

if(Group16.value=="Choice1"){

var S=this.getfield("Text21")

}

else{

var S=this.getfield("Text26")

}

}