Mathematically, the unit of length is:1km =1000m =10000m =100000mm =100000mm =/kloc-0. The width of a math textbook is only about 18 cm, which is 180 mm.

The following is an introduction to mathematical counting units:

The counting unit is the digital unit of measurement. We usually use decimal notation. The so-called "decimal" means that the relationship between every two adjacent counting units is: one large unit equals ten small units, that is, the forward speed between them is "ten".

Counting units shall include integer parts and decimal parts, arranged in the following order: Beijing, Gigabit, Hundred megabytes, Ten megabytes, Trillion, Billion, Billion, Million, Hundred Thousand, Hundred, Ten, One (1), One tenth, One hundredth and One thousandth.

China's figures of quantity are completely recorded in the ancient book Shu Shu Ji Yi. These figures include one, two, three, four, five, six, seven, eight, nine, ten, one hundred, one thousand, ten million, trillion, Beijing, Hong, Hong, Gou, frugal, upright and generous. The largest "pole" in China's numerical notation is not recorded in this book, but it is often used to express the concept of infinity.

Refer to Baidu Encyclopedia-Mathematical Counting Unit for the above information.