Mental Math Posted January 22, 2012 Share Posted January 22, 2012 (edited) You likely know the technique of multiplying any number by 11. Here is the trick to multiply a 2-digit and 3-digit number by 111. Multiplying a 2-digit number by 111 Example: 111 × 58 1. Write down the ones digit. > 8 2. Add the tens and ones; carry, if any. > 38 (5 + 8 = 13) 3. Repeat the previous step (and add carry, if any) > 438 (5 + 8 + 1 = 14) 4. Write the tens digit (add carry, if any) > 6438 (5 + 1 = 6) So, 111 × 58 = 6,438 Multiplying a 3-digit number by 111 Use right-to-left "sweeps" of 1, 2, 3, 2, and 1 digit. Example: 111 × 234 1. Write down the right most digit. > 4 2. Add the 2 right most digits (carry, if any) > 74 (3 + 4 = 7) 3. Add all the 3 digits (and carry, if any) > 974 (2 + 3 + 4 = 9) 4. Add the 2 left most digits (and carry, if any) > 5974 (2 + 3 = 5) 5. Write down the left most digit (add carry) > 25974 So, 111 × 234 = 25,974 Edited January 27, 2012 by swansont remove url Link to comment Share on other sites More sharing options...
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