## 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

Full List

### Multiplying two 2-digit numbers(difference of 4)

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

#### Example:

1. If the first number is 63, choose 67 as the second number.
2. The middle number (the average) is 65.
3. Square this middle number: 65 × 65 = 4225. (Remember how to square a 2-digit number ending in 5?)
4. Subtract 4 from this square: 4225 - 4 = 4221.
5. So 63 × 67 = 4221.

See the pattern?

1. If the first number is 38, choose 42 as the second number.
2. The middle number (the average) is 40.
3. Square this middle number: 40 × 40 = 1600.
4. Subtract 4 from this square: 1600 - 4 = 1596.
5. So 38 × 42 = 1596.

Choose the second number that will give you the easier square, and use your square shortcuts.