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.

So, how do you do three digit arithmetic on two hands? You could try using your nose. If it's just a little over two digits (one hundred and something), then consider your nose to be zero or one hundred. Consider 157 - 59. Set 57 on your hands in the usual way. Consider that your nose is set. You don't have nine to subtract, so use the ten's compliment, one. Add one and subtract ten. Well, you can add one. To subtract ten, you must add four and subtract five. Now you need to subtract 50. You don't have 50, so you must add 50 and subtract 100. Add the left thumb. Remember that 100 is on your nose. Unset your nose. Read the result: 098.

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 link provides two digit subtraction examples.

You might just want to go to the web site, so you can ask the generator yourself. Addition, subtraction or mixed problems, how many digits, more than just two numbers per problem? Answers on the same page or another page? Maybe you want one hundred pages to get you through vacation.

Remember to read the answers out loud. Practice every day that you eat. If you like, limit yourself to practice to those days of the week ending with 'y'. Enjoy yourself. With a little practice, two and three digit addition and subtraction can be highly reliable, fast and seemingly effortless.

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