5 cool math tricks ft. Technicality

Your demonstration for multiplying by eleven reminded me of a book my father brought home in the 60's. In the foreword it told of a small girl multiplying 735352314 by 11 who calculated the answer 8088875454 in less than a minute. A boy in the same class multiplied 5132437201 by 452736502785 to get the answer 2323641669144374104785 in seventy seconds. Both were students of the Trachtenberg system. The first thing I learned was to multiply by eleven and it uses the same concept of adding one digit of the number to the previous digit carrying when necessary. Very helpful in those days when a four function electronic calculator cost $400 and could only display 8 digits. BTW, the book is available online in PDF for those who are curious.