Take a 90-page A4 junior high school math book as an example. A math book is about 400 grams.

At the same time, because there are various versions of math books, A4 copy paper can be used as a reference to calculate the weight of math textbooks. The size of A4 copy paper is 21* 29.7cm (210mm× 297mm), and the size of an A4 copy paper is 0.210mm * 0.297mm = 0.06237m2.. If 70 grams of A4 paper per square meter weighs 70 grams, then the weight of an A4 copy paper is 70. People's Republic of China (PRC) national standard GB/T 148- 1997 "Paper size for printing, writing and drawing" is not equivalent to ISO2 16: 1975.