Mental arithmetic – I’ll keep it for teaching my kids later.

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Mental arithmetic – I’ll keep it for teaching my kids later. 1. Teens multiplied by teens: Mnemonic: Multiply the first digits, add the last digits, then multiply the last digits. Example: 12×14=? Solution: 1×1=1 　 2+4=6 　 2×4=8 　 12×14=168. Note: Multiply the units digits; if the result is not a two-digit number, use 0 as a placeholder. 　　2. Same head, complementary tails (sum of the tails equals 10): Mnemonic: Add 1 to the head, then multiply the head by itself and the tail by itself. Example: 23×27=? Solution: 2 + 1 = 3 　　2 × 3 = 6 　　3 × 7 = 21 　　23 × 27 = 621. Note: Multiply the units digits; if the result is not a two-digit number, use 0 as a placeholder. 　　3. The first multiplier is complementary, while the digits of the other multiplier are the same: Mnemonic: Add 1 to the first digit, then multiply the first digit by itself and the last digit by itself. Example: 37×44=? Solution: 3+1=4, 4×4=16, 7×4=28, 37×44=1628. Note: Multiply the units digits; if the result is not a two-digit number, use 0 as a placeholder. 　　4. Eleven times eleven: Mnemonic: 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 11 by any number: Rule: Keep the first and last digits unchanged, and add the middle digits together. Example: 11×23125=? Solution: 2+3=5, 3+1=4, 1+2=3, 2+5=7. 2 and 5 are at the beginning and end respectively. 11×23125=254375. Note: When the sum reaches 10, carry over 1. 　　6. Teens multiplied by any number: Mnemonic: Keep the first digit of the second multiplier unchanged and move down; multiply the units digit of the first factor by each digit following in the second factor, add the resulting number, then move down again. Example: 13×326=? Solution: For the units place, 3×3+2=11; 3×2+6=12; 3×6=18. Therefore, 13×326=4238. Note: When the sum reaches 10, carry over 1.