The problem here is that "miles" is a "lokal variable", which means that it can't be used in any other procedures or functions other than the MilesToKilometers one. To make this work, you need a global variable, here is an example:
SCAR Code:
program MilesToKilometers;
var
WriteMiles, WriteKilometers: Extended;
function MilesToKilometers(miles:extended):extended;
begin
result:= miles * 1.609;
WriteMiles := miles;
end;
procedure WriteFunction;
begin
WriteKilometers := MilesToKilometers(2.5); // Change it to whatever you want here ^^
if (WriteMiles = 1) then
begin
writeln(FloatToStr(WriteMiles)+' mile equals to '+floattostr(WriteKilometers)+' kilometers');
end else
begin
writeln(FloatToStr(WriteMiles)+' miles equal to '+floattostr(WriteKilometers)+' kilometers');
end;
end;
begin
writeFunction;
end.
This way it will also get the plural and singular right. 
If you need more explanation, let me know