Friday, August 04, 2006

Finger Multiplication Trick

When the student gets to multiplication, there is a problem. When learning addition, the student is tempted to count on their fingers to get the answer. For speed, the addition table should be memorized. Otherwise, they'll take quite a bit of time when it comes to 7 + 8.

For multiplication, figuring out 7 * 8 with repeated addition just doesn't work. And the third grader quickly memorizes the table.

There is a simple system for multiplication involving digits six and higher. And, it involves your fingers. Point your palms towards yourself with the finger tips of the hands pointing at each other. With the little fingers at the bottom, consider that your fingers represent the numbers six through ten. The little finger is six, the ring finger is seven, up to the thumb, which is ten.

We'll try 7 * 8 first. Seven is the ring finger of the left hand. Eight is the middle finger of the right hand. Order does not matter. Count the fingers to the bottom. That's 2 on the left hand and 3 on the right hand. 2 + 3 is 5. That's the ten's digit of 7 * 8. Examine the leftover fingers above. There are 3 on the left hand, and 2 on the right. Multiply these together. 2 * 3 = 6. That's the units digit. 7 * 8 = 56.

Consider 10 * 10. Ten fingers total below (and including) the thumbs. That's the ten's digit: 10. Zero fingers above. 0 * 0 = 0. That's the one's digit. 10 * 10 = 100.

I find myself somewhat reluctant to teach this tool. I mean, it works, yes. But does it accelerate the student, or retard progress? You decide.