The textbook "Mental Arithmetic: Addition & Subtraction, Part 3 - Big Friend" is an essential tool for mastering mental arithmetic. Through regular practice, students can enhance their arithmetic skills, sharpen concentration, improve memory, and stimulate both brain hemispheres. This comprehensive guide consists of 16 thoughtfully designed lessons, each focusing on a unique theme while progressively covering one-digit, two-digit, and three-digit calculations. In addition to arithmetic exercises, the textbook includes activities that support the development of fine motor skills and logical thinking. Ideal for both group and individual lessons, this textbook can be used with the guidance of a mental arithmetic teacher or for self-study at home. To maximize learning outcomes, it is recommended to pair this textbook with an Abacus and the Indigo Mental online platform https://indigomental.com or the mobile app Indigo Math Abacus for Kids. This will make the learning progresss more effective.