Multiplying by 9: The Nine Times Table Tricks

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What is the 'digit sum' trick for multiplying by $9$ (for numbers $1$-$10$)?

The two digits of the answer always add up to $9$. For example, $9 \times 4 = 36$, and $3 + 6 = 9$. Also, the first digit of the answer is always one less than the number you're multiplying by $9$. For $9 \times 4$, the first digit is $4-1=3$.

Use the 'digit sum' trick to find $9 \times 7$.

1. The first digit is one less than $7$, which is $6$. 2. The digits must add up to $9$, so $6 + ? = 9$. The second digit is $3$. 3. The answer is $63$.

$$ 9 \times 1 = ? $$

$$ 9 \times 1 = 9 $$

$$ 9 \times 2 = ? $$

$$ 9 \times 2 = 18 $$ (Note: $1+8=9$)

$$ 9 \times 3 = ? $$

$$ 9 \times 3 = 27 $$ (Note: $2+7=9$)

$$ 9 \times 4 = ? $$

$$ 9 \times 4 = 36 $$ (Note: $3+6=9$)

$$ 9 \times 5 = ? $$

$$ 9 \times 5 = 45 $$ (Note: $4+5=9$)

$$ 9 \times 6 = ? $$

$$ 9 \times 6 = 54 $$ (Note: $5+4=9$)

$$ 9 \times 7 = ? $$

$$ 9 \times 7 = 63 $$ (Note: $6+3=9$)

$$ 9 \times 8 = ? $$

$$ 9 \times 8 = 72 $$ (Note: $7+2=9$)

$$ 9 \times 9 = ? $$

$$ 9 \times 9 = 81 $$ (Note: $8+1=9$)

$$ 9 \times 10 = ? $$

$$ 9 \times 10 = 90 $$ (Note: $9+0=9$)

$$ 9 \times 11 = ? $$

$$ 9 \times 11 = 99 $$

$$ 9 \times 12 = ? $$

$$ 9 \times 12 = 108 $$