Interesting facts about mathematics

The presence of the letter a is a common characteristic of all odd numbers.

How many letters does the Roman numeral system consist of?

What are the letters of the Roman numeral system?（字母与字母间用顿号隔开）

The number which can be spelled with the same number of letters as itself is number

40 is the only number that is spelt with letters arranged in alphabetical order.

( ) is the only number that is spelt with letters arranged in descending order.

The opposite sides of a die always add up to( ).

When we write numbers from 0 to 1000, the letter a only appears first in 1000.