Total number of books = (120+100)/(1-45%) = 400 (Ben)

Number of sci-tech books =400×45%= 180 (book)

Method 2:

Let the number of science and technology books be x, then

x/45% =( 120+ 100)/( 1-45%)

X= 180

Total number of books = 180/45%=400 (volumes)

Method 3:

Let the total number of books be x and the number of science books be y, then

╭x+ 120+ 100=y ╭x= 180

{get {

45% Y = X = 400