Multiplying Two digit Numbers With difference of 6

1. Choose a 2-digit number.
2. Select a number either 6 smaller or 6 larger.
3. Find the middle number of the two (the average).
4. Square this middle number.
5. Subtract 9 from this square.

Example:

1. If the first number is 78, choose 72 as the second number.
2. The middle number (the average) is 75.
3. Square this middle number: 75 × 75 = 5625. (Remember how to square a 2-digit number ending in 5?)
4. Subtract 9 from this square: 5625 - 9 = 5616.
5. So 78 × 72 = 5616.