Chapter 13 on time and work talks a lot about how to be more efficient. If the amount of work done stays the same, then efficiency is the opposite of how long it takes to do the same amount of work. If it takes more time to do the work, it takes less time to do the work, and vice versa. All of these ideas are explained in detail in the study guides that Utopper gives to students who use RS Aggarwal class 8 books.