The multiples of 9 in 1- 100 are 9, 18, 27, 36, 45, 54, 63, 72, 8 1, 90, 99:7, respectively. Because books must be integers, Cong Cong's books must be divisible by 9, and obvious books must be divisible by 7, so ... Cong Cong's 72 books are obviously 28, and Cong Cong donated 32 books and obviously donated 8 books!