Let the number of chemistry books be x and the number of calculus books be y total number of books in each case is: x+y=24 x=24-y.....i
Weight of each case: 2707.2/20 =135.36 thus 4.2x+6.0y=135.36..ii substituting i in ii 4.2(24-y)+6y=135.36 100.8-4.2y+6y=135.36 hence 1.8y=34.56 y=19.2 hence x=24-19.2 x=4.8 thus the number of chemistry books was 4.8*20=96 number of calculus was 19.2*20 =384 books