"Scrisoarea ta este mai scurtă decât a mea!"

Read this thread, user Shattenparker elaborates on the differences: https://www.duolingo.com/comment/21783519

If link breaks, I'll quote him here:

"no, if you Google it, you'll find that only decât is correct for inegality, but apparently Romanians use ca anyway. (Same is true in German, where it is supposed to be "größer als", but regionally people say "größer wie" anyway). And you'll even find English speakers who falsely say "taller as" - so this logical problem seems to be quite universal..." -Shattenparker

and later on probably I was there someone else had asked but I just started this lesson so had no clue, all in all thanks : )