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By Martin I.M. Andrew

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If you want to multiply 4 by 11, you simply repeat the digit for the answer, 44. If you multiply 6 by 11 you get 66. What answer do you get if you multiply 9 by 11? Easy, 99. Did you know it is just as easy to multiply two-digit numbers by 11? Multiplying a two-digit number by 11 To multiply 14 by 11 you simply add the digits of the number, 1 + 4 = 5, and place the answer in between the digits for the answer, in this case giving you 154. Let’s try another: 23 × 11. 2+3=5 Answer: 253 These calculations are easy to do in your head.

640 − 28 = 612 To subtract 28, we subtract 30 and add 2. 640 − 30 = 610 610 + 2 = 612 Our problem now looks like this: Let’s check the answer by casting out the nines: Zero times 7 is 0, so the answer is correct. Using 50 as a reference number That takes care of the numbers up to around 30 times 30. What if the numbers are higher? Then we can use 50 as a reference number. It is easy to multiply by 50 because 50 is half of 100. You could say 50 is 100 divided by 2. So, to multiply by 50, we multiply the number by 100 and then divide the answer by 2.

Let’s try another number. Three nines are 27. Add the digits of the answer together, 2 + 7, and you get 9 again. Eleven nines are 99. Nine plus 9 equals 18. Wrong answer? No, not yet. Eighteen is a two-digit number so we add its digits together: 1 + 8. Again, the answer is 9. If you multiply any number by 9, the sum of the digits in the answer will always add up to 9 if you keep adding the digits until you get a one-digit number. This is an easy way to tell if a number is evenly divisible by 9.

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