## BEATCALC:  Beat the Calculator!

Back to Calculation Tips & Tricks

 From B. Lee Clay

Squaring Numbers

Multiplying Numbers

Dividing Numbers

Subtracting Numbers

Finding Percents

Calculation
Practice Exercises

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### Multiplying a 2-digit number by 891

1. Select a 2-digit number.
2. Multiply it by 9.
3. If the product is 2 digits, subtract 1:
X X _ _
Then subtract the product from 100:
_ _ X X
4. If the product is 3 digits, add 1 to the first digit and
subtract it from the product: X X X _ _
Then subtract the last 2 digits of the product from 100: _ _ _ X X

#### Example:

1. The 2-digit number chosen to multiply by 891 is 11.
2. Multiply by 9: 9 × 11 = 99
3. Subtract 1: 99 - 1 = 98: 9 8 _ _
4. Subtract the product from 100: 100 - 99 = 1:
_ _ 0 1
5. So 11 × 891 = 9,801.

See the pattern?

1. The 2-digit number chosen to multiply by 891 is 34.
2. Multiply by 9: 9 × 34 = 270 + 36 = 306
3. Add 1 to the first digit: 3 + 1 = 4
4. Subtract 4 from the product: 306 - 4 = 302:
3 0 2 _ _
5. Subtract the last 2 digits from 100: 100 - 6 = 94:
_ _ 9 4
6. So 34 × 891 = 30,294.