Multiplying Two digit Numbers Having 1st digit same1

1. Select two 2-digit numbers with the same first digit.
2. Multiply their second digits (keep the carry). _ _ _ X
3. Multiply the sum of the second digits by the first digit, add the carry (keep the carry). _ _ X _
4. Multiply the first digits (add the carry). X X _ _

Example:

1. If the first number is 42, choose 45 as the second number (any 2-digit number with first digit 4).
2. Multiply the last digits: 2 × 5 = 10 (keep carry)
   _ _ _ 0
3. Multiply the sum of the 2nd digits by the first:
   5 + 2 = 7; 7 × 4 = 28; 28 + 1 = 29 (keep carry)
   _ _ 9 _
4. Multiply the first digits (add the carry)
   4 × 4 = 16; 16 + 2 = 18
   1 8 _ _
5. So 42 × 45 = 1890.