Wow, this is too amazing! I'll keep it and teach my kids in the future!!!
Hong Tao
1. Multiplication of numbers in the teens: Rule of thumb: Multiply the first digits, add the last digits, multiply the last digits.
Example: 12×14=? Solution: 1×1=1, 2+4=6, 2×4=8 → 12×14=168
Note: If the multiplication of the last digits does not reach two digits, use 0 as a placeholder.
2. Same first digit, complementary last digits (sum of last digits equals 10): Rule of thumb: Add 1 to one of the first digits, then multiply the first digits and multiply the last digits.
Example: 23×27=? Solution: 2+1=3, 2×3=6, 3×7=21 → 23×27=621
Note: If the multiplication of the last digits does not reach two digits, use 0 as a placeholder.
3. The first multiplier is complementary, the second multiplier has identical digits: Rule of thumb: Add 1 to one of the first digits, then multiply the first digits and multiply the last digits.
Example: 37×44=? Solution: 3+1=4, 4×4=16, 7×4=28 → 37×44=1628
Note: If the multiplication of the last digits does not reach two digits, use 0 as a placeholder.
4. Numbers ending in "one" multiplied by each other: Rule of thumb: Multiply the first digits, add the first digits, multiply the last digits.
Example: 21×41=? Solution: 2×4=8, 2+4=6, 1×1=1 → 21×41=861
5. Multiplying any number by 11: Rule of thumb: Keep the first and last digits the same, sum the intermediate digits.
Example: 11×23125=? Solution: 2+3=5, 3+1=4, 1+2=3, 2+5=7, with 2 and 5 at the start and end respectively → 11×23125=254375
Note: If the sum reaches ten, carry over.
6. Multiplying a number in the teens by any number: Rule of thumb: Keep the first digit of the second multiplier unchanged, multiply the unit digit of the first factor by each digit of the second multiplier, add the next digit, and then place it down.
Example: 13×326=? Solution: The unit digit of 13 is 3, 3×3+2=11, 3×2+6=12, 3×6=18 → 13×326=4238
Note: If the sum reaches ten, carry over.
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