Let x be the number of dozens of exercise books bought. Then the number of books bought was 12x. The number of books sold is (12x - 13). The money equation for the profit is easy to write. It is (12x-13)*32 = 12x*20 + 88. From this equation 12x*32 - 12x*20 = 88 + 13*32, or (12*32 - 12*20)x = 504 144x = 504 x == 3.5. ANSWER. 3.5 dozens of books were bought. // 3.5 dozens is 12*3.5 = 42 books, so there is no reason to worry about the integer number of books.