## BEATCALC:  Beat the Calculator!

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 From B. Lee Clay

Squaring Numbers

Multiplying Numbers

Dividing Numbers

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### Multiplying a 2-digit number by 11 1/2

1. Select a 2-digit number.
2. Divide by 2.
3. To this, add 11 times the original number.

#### Example:

1. The 2-digit number chosen to multiply by 11 1/2 is 21.
2. Divide by 2: 21/2 = 10 1/2.
3. To this, add 11 times original number: 10 1/2 + 11(21) = 10 1/2 + 231 = 241 1/2.
4. So 11 1/2 × 21 = 241 1/2.
Recall that to multiply a number by 11 (for example, 11 × 21), the pattern is:
1. Last digit is second digit of number (continuing the example above, 1)
2. Middle digit is sum of the numbers' two digits (2 + 1 = 3)
3. First digit is first digit of number (2)
4. So 11 × 21 is 231.

See the pattern?

1. The 2-digit number chosen to multiply by 11 1/2 is 85.
2. Divide by 2: 85/2 = 42 1/2.
3. To this, add 11 times original number: 42 1/2 + 11(85) = 42 1/2 + 935 = 975 + 2 1/2 = 977 1/2.
4. So 85 × 11 1/2 = 977 1/2.
Practice multiplying by 11. Remember that when sum of the two digits is 10 or more, add one to the first digit for the first digit of the product.