The Russian number ‘40’ came from Old Rus trade

Unlike other names for numbers (‘pyat'desyat’ – ‘fifty’, ‘shestdesyat’ – ‘sixty’, ‘sem'desyat’ – ‘seventy’), ‘forty’ (40) is not formed from a combination of ‘four’ (4) + ‘ten’ (10), but has its own unique history. The word ‘sorok’ is connected to the Old Rus fur trade. It referred to a bundle of 40 sable skins – exactly the number needed to make one fur coat.

The word ‘sorok’ then replaced the name ‘chetyre desyate’ (‘four tens’) for being more convenient in speech. This word exists only in Russian, Ukrainian and Belarusian. Other Slavic languages still use the ‘4’ + ‘10’ combination.