Multiply any 2-digit number by 11 in 3 seconds

The trick in one sentence

To multiply a 2-digit number by 11: add the two digits, then slide the result in the middle.

A snappy example

Take $34 \times 11$:

1. The two digits of 34 are 3 and 4.
2. Add them: $3 + 4 = 7$.
3. Slide this 7 between the 3 and the 4 → 374.

Check: $34 \times 11 = 374$ ✓

Another: $52 \times 11$

• Digits: 5 and 2
• Sum: $5 + 2 = 7$
• Result: 572

What if the sum exceeds 9?

Take $78 \times 11$. The sum $7 + 8 = 15$ doesn’t fit in one digit. Then:

• Write 5 in the middle
• Carry the 1 to the left digit: $7 + 1 = 8$
• Result: 858

Check: $78 \times 11 = 858$ ✓

Why it works

Multiplying by 11 is multiplying by $10 + 1$. So:

$\overline{ab} \times 11 = \overline{ab} \times 10 + \overline{ab} = \overline{ab0} + \overline{ab}$

When you write the addition, the middle digit becomes $a + b$ — exactly the trick!

?Your turn

Your turn: what is 63 × 11?

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?Your turn

What is 87 × 11?

A.857B.957C.877D.997

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