## Tuesday, August 08, 2006

### Finger Arithmetic Part Ten

Continuing finger arithmetic from Part Nine. This one is called Part Ten. It is the eleventh part in the series. It continues subtraction with two digit subtraction.

Some two digit subtraction is easy. Consider 34 - 12. Set 34. To subtract two, remove two fingers on the right hand. To subtract ten, remove one finger from the left hand. Read the answer: 22.

Sometimes, you need to borrow from the next digit up. For example, 34 - 16.
Set 34 on your hands. Now, six is the same as five plus one. That is, it is a thumb and a finger. You can subtract a finger from your right hand. You do not have a thumb to subtract. You can get where you want to go by considering that subtracting five is the same as adding five and subtracting 10: -5 = + 5 - 10. So, add a thumb to the right hand and subtract a finger from your left hand. Now you just need to subtract the ten of 16. Subtract another finger from your left hand. Read the answer. 18.

Next up is 31 - 8. This time, the ten's compliment is used. Set 31. Recognize that there is no way to subtract any part of 8 from it. Not 8, not 3. Consider than subtracting eight is the same as adding 2 and subtracting 10. -8 = +2 - 10. So, add two fingers to the right hand, and subtract one from the left hand. Read the answer: 23.