The tens follow a vigesimal system from forty up, with the exception of fifty:
A modern national language which uses a full vigesimal system is Dzongkha in Bhutan.
The vigesimal system remains in robust use.
This arithmetic tool was based on the vigesimal system (base 20).
Vennemann regards the vigesimal system as a trait of the Vasconic language.
The ancient Mayan numerical system - counting on fingers and toes - was a base 20 or "vigesimal" system.
The Nordic vigesimal system may possibly derive ultimately from the Celtic.
A vigesimal system with 5 as a sub-base is found in Nahuatl and the Maya numerals.
Dzongkha, the national language of Bhutan, has a full vigesimal system, with numerals for the powers of twenty 20, 400, 8,000, and 160,000.
The Kharosthi numeral system behaves like a partial vigesimal system.
