The mayans' vigesimal numbering system was based on
30
20
10
5

The Mayans' vigesimal numbering system was based on 20.

The Maya constructed a very sophisticated number system because they needed it for astronomy and calendar calculations.

The Maya system was vigesimal ( based on the number 20, probably developed from counting on fingers and toes ). The numerals consisted of only three symbols : zero ( shell shape ), one ( a dot ) and five ( a bar ).  Addition and substation was a matter of adding up dots and bars. Numbers after 19 were written vertically in powers of twenty ( the Arabic numeral system uses the powers of 10 ).

The Maya mathematics had no influence on the European and Asian numbering system due to the geographical disconnect of the Maya.

1. it was caused by feudalism

2. charlemagne used christianity as a major cornerstone for his rule. cloisters were built all over the empire to act as centers for the (re)population of the vast european wilderness.

3. the europeans feared the next raid every time chetak

i'm not so sure about #6. but i hope i you with the first 3.