Multiplying 2 2-digit numbers with the sum of the units digits equal to 10, and the tens digits are identical
Example 1. 67 x 63
Method
Step 1. The last digits of the answer are the product of units digits, that is, 7 x 3 = 21
Step 2. Add 1 to the tens digit, then multiply by the original units digit, that is, (6 + 1) x 6 = 7 x 6 = 42
Step 3. Place the answer in 2 in front of answer in 1, the final answer is 4221
Example 2. 39 x 31
Step 1. 9 x 1 = 09
Step 2. (3 + 1) x 3 = 4 x 3 = 12
Step 3. 1209
