Why can't I write inches in decimal form?

If I use Mathcad 15, I can define a variable such as "a:= .7 in"  if I write this exact same thing in MathCad prime it will result in an error.  I have to write "a:= 7 in" then come back and add the decimal place after it's accepted 7 inches.  What am I doing wrong? This seems absurd and is incredibly frustrating.