Download Speed Mathematics: Secrets of Lightning Mental Calculation by Bill Handley PDF

By Bill Handley

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Additional info for Speed Mathematics: Secrets of Lightning Mental Calculation

Sample text

We use a reference number of 10. Both numbers are above 10 so we draw the circles above. Twelve is 2 above 10, 21 is 11 above so we write 2 and 11 in the circles. Twenty-one plus 2 is 23, times 10 is 230. Two times 11 is 22, added to 230 makes 252. This is how your completed problem should look: Multiplying numbers above 100 Can we use this method for multiplying numbers above 100? By all means. To multiply 106 by 104, we would use 100 as our reference number. The numbers are above the reference number, 100, so we draw circles above 106 and 104.

Let’s try it again, this time using 13 × 15: 13 × 15 = 195 1 + 3 = 4 (substitute for 13) 1 + 5 = 6 (substitute for 15) 4 × 6 = 24 Twenty-four is a two-digit number so we add its digits. 2 + 4 = 6 Six is our check answer. Now, to find out if we have the correct answer, we check this against our real answer, 195. 1 + 9 + 5 = 15 Bring 15 to a one-digit number: 1 + 5 = 6 Six is what we got for our check answer so we can be confident we didn’t make a mistake. Casting out nines There is another shortcut to this procedure.

Substitute numbers To check a calculation, we use substitute numbers instead of the real numbers we were working with. A substitute on a football or basketball team is somebody who replaces somebody else on the team; they take another person’s place. That’s what we do with the numbers in our problem. We use substitute numbers instead of the real numbers to check our work. Let’s try an example. Let us say we have just calculated 13 times 14 and got an answer of 182. We want to check our answer. 13 × 14 = 182 The first number is 13.

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