## BEATCALC:  Beat the Calculator!

Back to Calculation Tips & Tricks

 From B. Lee Clay

Squaring Numbers

Multiplying Numbers

Dividing Numbers

Adding Numbers

Subtracting Numbers

Finding Percents

Calculation
Practice Exercises

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### Multiplying a 4-digit number by 99

1. Select a 4-digit number.
2. Subtract the first two digits plus 1 from the number.
X X X X _ _
3. Subtract the last two digits of the number from 100.
_ _ _ _ X X

#### Easier example:

1. Choose a 4-digit with digits from smaller to larger: 2368.
2. The first two digits will be the same: 2 3 _ _ _ _.
3. Subtract the first two digits + 1 from the last two digits: 23 + 1 = 24, 68 - 24 = 44: _ _ 4 4 _ _
4. Subtract the last two digits from 100: 100 - 68 = 32:
_ _ _ _ 3 2
5. So 2368 × 99 = 234432.

See the pattern?

#### Advanced example:

1. The 4-digit number chosen to multiply by 99 is 3512.
2. Subtract the first two digits + 1 from the number:
35 + 1 = 36, 3512 - 36 = 3512 - 30 - 6 = 3482 - 6
= 3476: 3 4 7 6 _ _
3. Subtract the last two digits from 100: 100 - 12 = 88:
_ _ _ _ 8 8
4. So 3512 × 99 = 347688.

Start out with easier examples and graduate to more difficult ones. Remember to subtract from left to right in increments.

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