Voted Best Answer
May 18, 2013 - 10:15 PM
The "twenties" are an exception -- I think because the "e" sound at the end of "veinte" is similar to the "y" in numbers like veinte y seis (etc). The same can't be said for treinta, cuarenta, cinquenta, etc. because those end with "a." So, you can combine the three words (e.g. veinte y siete) into one (veintisiete) for the 20s, but not anything else (treinta y siete is always just that).