Multiplying Two digit Numbers With 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.