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Lesson 5 MORE ELEMENTARY ADDITIONMental calculationWe now begin the four problems of arithmetic: Addition, subtraction, multiplication, and division. They are also called the four operations. We continue to emphasize problems that do not require a calculator -- problems that you should not even have to write!
In this Lesson, we will answer the following:
The student should now have mastered the elementary addition of one-digit numbers, and should be practicing the multiplication table. For, once you know that
These are not problems to write in a column. Our first skill is based on the following properties of addition:
Essential for mental calculation is knowing the multiples of 10: 10, 20, 30, 40, 50, and so on. |
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Example 1. 25 + 7 = (25 + 5) + 2 = 30 + 2 = 32.
Similarly, 38 + 4 = (38 + 2) + 2 = 42. 59 + 7 = (59 + 1) + 6 = 66. 96 + 8 = (96 + 4) + 4 = 104. To add 96 + 8, then, an educated person does not have to take out a pencil, write in a column and say, 
"6 plus 8 is 14 -- write 4, carry 1. etc., etc., etc." Nor does an educated person count on her fingers. Rather, an educated person knows elementary addition, and therefore that 96 + 8 is not very different from 6 + 8. It ends in 4. Example 2. Since 9 + 6 = 15, then
A 9 plus a 6 always gives a "15." This brings us to the practical technique for adding several numbers -- 7 + 8 + 3 + 9 + 4 Adding by endings. |
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For example, since 8 + 4 = 12, you would know that if you add any number ending in 8 to any number ending in 4, the sum will always end in 2. 28 + 4 = 32 38 + 4 = 42 68 + 4 = 72 And so on. Each answer falls in the next decade. Similarly, since 6 + 5 = 11 then 26 + 5 = 31 46 + 5 = 51 76 + 5 = 81 They all end in 1. These are not problems to write down. Example 3. Add mentally -- or aloud -- from left to right.
Say only the partial sum as you come to it. Do not say,
Two numbers are called complementary if their sum is 10. And since the order of terms does not matter, we may take advantage of complements.
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435 + 461 = 896 First add the hundreds, then the tens, then the ones. We now see the fundamental principle of all mental calculation: Calculate from left to right, just as you read. Example 5. Counting by 10's. 30 + 24 = 54.
Here are other examples: 20 + 16 = 36 40 + 38 = 78 40 + 62 = 102 40 + 82 = 122 90 + 73 = 163 Example 6. 43 + 25 First add the tens then the ones. Say "Sixty -- 43 + 25 -- eight." Or, you could say, "43 plus 20 is 63, plus 5 is 68." 
Say, "150 plus 7 is 157."
Say only, "50 plus 14 is 64." The art of mental calculation is to say as little as possible. The last number you say is the answer.
"170 plus 11 is 181." Example 10. 23 + 32 + 25 + 12 First add all the tens, then add on the ones. As you add each place, say the partial sum. Say 
The last number you say is the entire sum.
Example 12. 653 + 224 First add the hundreds, then the tens, then the ones. Again, say each partial sum: 653 + 224 Say, "800, 870, 877." Example 13. Three tracks on a CD have the following times: 10:34 6:25 8:07 What is the total time? (10:34 means 10 minutes 34 seconds. 60 seconds = 1 minute. Therefore, 72 seconds = 1 minute 12 seconds. 1:12.) Technique. Start with the minutes and count: "16 plus 8 is 24 minutes." Now add on the seconds. "24:59 plus 7 is 24: 66." The total time is 25 minutes 6 seconds. At this point, please "turn" the page and do some Problems. or Continue on to Section 2: | ||||||||||||||||||||||||||||||||||||||||||||||
