This book is the second in the series â€œThe adventurers of the numbersâ€ In this book, all of the numbers are traveling the world and having math exercises together. This is the version of Subtraction. The book is for children from ages four to seven, who are beginning to learn about numbers. This book makes math more fun for kids. The book is an adventure of the numbers. It is an educational book that shows that math can be fun. Â The book also can be read as a bedtime story.