Consider the question. A new book sold 15 copies at the bookstore on Wednesday. On Thursday the book sold three times that many copies. On Friday the book sold one-fifth the number of copies it did on Thursday. How many copies of the book did the bookstore sell by Friday?
 What is the step-by-step reasoning process to arrive at the answer: 69?
On Thursday 15 * 3 = 45 books were sold. On Friday 45 / 5 = 9 books were sold. Adding up all three days 15 + 45 + 9 = 69 books were sold by Friday.