Librería Samer Atenea
Kálamo Books
Librería Elías (Asturias)
Librería Kolima (Madrid)
Librería Proteo (Málaga)
Many useful procedures explained and taught: 2-column addition, left-to-right subtraction, direct multiplication by numbers greater than 12, mental division of large numbers, more. Also numerous helpful shortcuts. More than 8,000 problems, with solutions.