The explanations of this book primarily use the logical steps to let users easily memorize the methods of solving questions. The essence of mathematics is logic. Human thinking naturally accepts the formula of "because" and "therefore." Hence, using mathematically logical steps to solve questions directly is in harmony with the natural function of human brain. Furthermore, it is helpful for memorizing because what you need to remember are the mathematically logical steps, which is refined and easily to be memorized, instead of the mixed explanations of English and math that are difficult to memorize. To save readers' time, this set of books uses the method, at the solution part, of completely replicating the original questions. The purpose of this design is to save time for readers from flipping back and forth through the pages to see the accurate descriptions of the original question. Even though the cost for the books will increase with such design, namely, it will cost you more, yet, it saves you enormous and precious amount of time.