With four subjects, completing one unit per subject each day, you can finish one book in about 5 days. Please study with easy-to-understand materials this winter break. Although they are thin, Z-kai has a long-standing reputation. The high monthly tuition was a drawback. Some people still buy them even if they have writing in them. However, it's a shame when there's writing, as it demotivates children. Haha. It seems they get tired of thick workbooks. Compared to problem books in bookstores, they are clearly thinner, but Z-kai's explanations are very easy to understand. The concept of fractions was explained using easy-to-understand ways of cutting a whole cake. Even I, an adult, thought, "I see!" If you have about 3 books, you'll be able to solve a lot of good problems. Since there's no writing, you can use them to your heart's content. The monthly fee was 7,000 yen, but it's a bargain at 1,750 yen per subject. There's more, so please try it out first. With 3 books, you can probably improve your academic ability during the winter break without overdoing it. Good luck!