In the movie Stand and Deliver (Warner Brothers 1988), Edward James Olmos,
portraying mathematics teacher Jaime Escalante, confronts Chuco, a defiant
gang member, during Olmos's class at Garfield High School in East Los Angeles:
Escalante: Ohh. You know the times tables?
Chuco: I know the ones. ..twos. ..three.
[On "three" Chuco flips the bird to Escalante.]
Escalante: Finger Man. I heard about you. Are you The Finger Man?
I'm the Finger Man, too. Do you know what I can do?
I know how to multiply by nine! Nine times three.
What you got? Twenty-seven. Six times nine.
One, two, three, four, five, six. What you got? Fifty-four.
You wanna hard one? How about eight times nine?
One, two, three, four, five, six, seven, eight.
What do you got? Seventy-two. (Warner Brothers 1988)
To capture Chuco's attention, Escalante was using a well-known
finger-multiplication trick: multiplying by nines by counting on his fingers.
The method that Escalante used with Chuco for finding multiples of nine
up to 90 -- requires that the students hold both hands up, with palms facing
the student. The student counts his or her thumbs and fingers consecutively
from left to right, with the thumb on the left hand representing the number 1
and the thumb on the right hand representing the number 10. To multiply n
times 9, where 1 <= n <= 10 and where n is a whole number, the student bends
down the nth finger. The number of fingers to the left of the bent finger
represents the tens-place digit of the product, whereas the number of fingers
to the right of the bent finger represents the ones-place digit of the product.
The figure below illustrates 7 x 9. Mr. Kolpas explains why this works in his
article in the April 2002 issue of the Mathematics Teacher.