Multiplying two 2-digit numbers

steps

1. Both numbers should have the same first digit।
2. Choose second digits whose sum is 10.
3. Multiply the first digit by one number greater than itself; this number will be the first part of the answer:
   X X _ _.
4. Multiply the two second digits together; the product
   will be the last part of the answer: _ _ X X.

Note: If the two second digits are 1 and 9 (or, more generally, have a product that is less than ten), insert a 0 (zero) for the first X in step 4.

(Thanks to Michael Richardson, age 10, for this note.)

Example:

1. If the first number is 47, choose 43 as the second number (same first digit, second digits add to 10).
2. 4 × 5 = 20 (multiply the first digit by one number greater than itself): the first part of the answer is
   2 0 _ _.
3. 7 × 3 = 21 (multiply the two second digits together); the last part of the answer is _ _ 2 1.
4. So 47 × 43 = 2021.

See the pattern?

1. If the first number is 62, choose 68 as the second number (same first digit, second digits add to 10).
2. 6 × 7 = 42 (multiply the first digit by one greater), the first part of the answer is 4 2 _ _.
3. 2 × 8 = 16 (multiply the two second digits together); the last part of the answer is _ _ 1 6.
4. So 62 × 68 = 4216.