## Wednesday, July 12, 2006

### Finger Arithmetic Part Eight

Continuing finger arithmetic from Part Seven. This one is called Part Eight. It is the ninth part in the series.

Back in Part Seven, we finished up addition of single digit numbers. We took care of carries with Ten's Compliments and Five's Compliments. In this lesson, we continue forward with two digit addition.

Consider 19 + 27. Set nineteen by making two fists, setting one finger on the left hand and all fingers and thumb on the right hand. To add seven, note that seven is five plus two. There aren't two fingers to add, so you use Ten's Compliment. Subtract the Ten's Compliment of seven, which is three, and add ten, which is one finger on the left hand. To add twenty, add two more fingers on the left hand. Read the answer: 46. Use Ten's Compliments for numbers over five, and Five's Compliments for numbers less than five.

In the previous example, we used the Five's Compliment of three to add it to the existing seven. There were a number of confusing steps to get through adding just the one digit. While this method works, you could instead use the Ten's Compliment of three, which is seven. Here's how that works. Set thirty seven on your hands. To add three, subtract the Ten's Compliment of three, which is seven, and add ten. You can add the ten, so you just do it. I find it conceptually easier to use Five's Compliments for digits less than five, and Ten's Compliments for digits more than five. This always works, though it leads to extra steps now and then. As your proficiency improves, feel free to experiment with Ten's Compliments where they make sense.

You might spend a few weeks on this before moving on to subtraction.

Again, you are referred to the example generator, which can provide you with an infinite number of examples. Each time you click here, you get a new page. Use your back button to get back to the lesson. This isn't the same link as in the Part Six. This link provides two digit examples.

The example generator is the same, with different parameters. This is the form that lets you select the parameters you might want. For example, if you have a Japanese Abacus, called a Soroban, you'll have more than two hands, as each hand is basically one rod of this device. Let's say that you have a child in elementary school. If they're reading for enjoyment, then handing them some books to read over the summer will allow them to advance, or at least not backslide too much. What about arithmetic? It is estimated that children backslide an average of about two and a half months over the summer. This problem generator can help. I try to work in five minutes with my son every day. There is also a problem generator for multiplication on my site. Feel free to use it for division problems as well.