Multiplying two 2-digit numbers (same 1st digit, 2nd digits sum to 10)

Step 1:Choose second digits whose sum is 10.
Step 2:Multiply the first digit by one number greater than itself; this number will be the first part of the answer:
X X _ _.
Step 3: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.



Example:
If the first number is 47, choose 43 as the second number (same first digit, second digits add to 10).
4 × 5 = 20 (multiply the first digit by one number greater than itself): the first part of the answer is 2 0 _ _. 7 × 3 = 21 (multiply the two second digits together); the last part of the answer is _ _ 2 1.
So 47 × 43 = 2021.
 
Example 2:
If the first number is 62, choose 68 as the second number (same first digit, second digits add to 10).
6 × 7 = 42 (multiply the first digit by one greater), the first part of the answer is 4 2 _ _.
2 × 8 = 16 (multiply the two second digits together); the last part of the answer is _ _ 1 6.
So 62 × 68 = 4216.