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