Supplemental textbook = 2100 yen ($21.43 €16.41)

There is no charge for school textbooks. But my daughter is not smart, especially at science. The textbook alone is not enough for her to understand science. So she needs a supplemental textbook to comprehend science. It cost 2100 yen ($21.43 €16.41). That supplemental textbook explains closely what the primary textbook wants to say, and sometimes tells you the answer of the drill in the primary textbook. I think the answer part is helpful for lower level students.

The right (upper) book is the supplemental textbook. The left one is a drill book, costs 1340 yen ($13.6 €10.47).