"The number is forty."
Translation:Το νούμερο είναι σαράντα.
Yes, actually in Medival/Byzantine Greek. The τες were considered as an article then (maybe because of the large mass of uneducated people) as if the word were τες σαράκοντα. So it was dropped. Then, in Modern Greek, came another change: The drop of the -κο- inside the arithmetic words. So σαράντα<σαράκοντα, τριάντα<τριάκοντα, πενήντα<πεντήκοντα etc. Those forms with -κο- may be still used rarely in very formal and usually of linguistic importance texts.