## 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 two 2-digit numbers(difference of 10)

1. Choose a 2-digit number.
2. Select a number either 10 smaller or 10 larger.
3. Find the middle number of the two (the average).
4. Square this middle number (multiply it by itself).
5. Subtract 25 from this square.

#### Example:

1. If the first number is 36, choose 26 as the second number (10 smaller).
2. The middle number (the average) is 31.
3. Square this middle number: 31 × 31 = 961. (Remember how to square a 2-digit number ending in 1?)
4. Subtract 25 from this square: 961 - 25 = 936
(subtract mentally in steps: think 961 - 20 - 5 = 941 - 5 = 936).
5. So 36 × 26 = 936.

See the pattern?

1. If the first number is 78, you might pick 88 as the second number (10 larger).
2. The middle number (the average) is 83.
3. Square this middle number: 83 × 83 = 6889. (Remember how to square a 2-digit number ending in 3?)
4. Subtract 25 from this square: 6889 - 25 = 6864
(subtract mentally in steps: think 6889 - 20 - 5 = 6869 - 5 = 6864).
5. So 78 × 88 = 6864.

Remember to subtract in easy steps and pick your number to get an easy square. With practice you will be a whiz at getting these products.