6 decimeters =60 centimeters =600 millimeters. Of course, a math book can't be equal to 6 decimeters or 6 centimeters, so the thickness of a math book is about 6 mm.

This question examines the knowledge points of length units.

The unit of length refers to the basic unit for regulating the length. Its international unit is "meter" (symbol "m"), and commonly used units are millimeter (mm), centimeter (cm), decimeter (dm), kilometer (km), meter (m), micron (micron) and nanometer (nm). The basic transformation relationship is as follows:

1 km =1000m =10000m =100000mm =100000mm.

So the thickness of a math book is about 6 mm

Other commonly used units of length are:

(1) km

Kilometers, abbreviated as "km", are usually used to measure the distance between two places. The commonly used conversion relationship is as follows: 1 km (km) = 1 0,000m (m) = 1 00,000m (cm) =10,000mm (mm).

(2) meters

The unit of length "meter" in the international system of units originated in France. 1790 In May, a special committee composed of French scientists suggested that one fortieth of the total length of the earth meridian passing through Paris should be used as the unit of length-meter, and this proposal was approved by the French Parliament in 179 1 year.

In order to make a benchmark to represent the value of rice, under the leadership of French astronomers Gerliangbule and Mishin, measurements were made from Dunkirk, France to Barcelona, Spain, and from 1792 to 1799. 1799, according to the measurement results, a short-section platinum rod of 3.5 mm× 25 mm was made, and the distance between the two ends of the rod was set as 1 m, which was kept by the French archives, so it was also called "archive meter". This is the earliest definition of rice.

(3) decimeter

Decimeter or dm is one of the metric units of length, and 1 decimeter is equivalent to one tenth of 1 meter. The common conversion relationship is as follows: 1 decimeter = 0.000 1 kilometer (km) = 0. 1 meter (m) = 10 centimeter (cm) = 100 millimeter (mm).

4) cm

Centimeter, unit of length; Chinese: centimeter (s), abbreviated as cm. The unit conversion of centimeters is as follows:1cm =10 mm = 0.1decimeter = 0.0 1 m = 0.0000 1 km.

(5) mm

Millimeter, also known as millimeter (or millimeter), is a unit of length and rainfall, and the symbol is ㎜. 1mm is equivalent to one thousandth of 1mm (this means "milli"). The decimal relation is as follows:1mm = 0.1cm = 0.01decimeter = 0.001m = 0.000001km.